Process for the energy-efficient production of alkali metal alkoxides

ABSTRACT

A process produces sodium and/or potassium alkoxides in countercurrent by reactive rectification. Alcohol is reacted in countercurrent with the respective alkali metal hydroxide. The vapours containing alcohol and water are separated into at least two serially arranged rectification columns. The energy of the vapour obtained in the first rectification is utilized for operating the second rectification. This specific energy integration coupled with establishing a certain pressure difference in the two rectification stages makes it possible to cover a particularly large proportion of the energy required for the rectification through electricity and to save heating steam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Application No. 21168921.1, filed on Apr. 16, 2021, the content of which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a process for producing sodium and/or potassium alkoxides in countercurrent by reactive rectification. Alcohol is reacted in countercurrent with the respective alkali metal hydroxide. The vapours comprising alcohol and water are separated into at least two serially arranged rectification columns. The energy of the vapours obtained in the first rectification is utilized for operating the second rectification. This specific energy integration coupled with establishing a certain pressure difference in the two rectification stages makes it possible to cover a particularly large proportion of the energy required for the rectification through electricity and to save heating steam.

Description of Related Art

The production of alkali metal alkoxides is an important industrial process.

Alkali metal alkoxides are used as strong bases in the synthesis of numerous chemicals, for example in the production of pharmaceutical or agrochemical active ingredients. Alkali metal alkoxides are also used as catalysts in transesterification and amidation reactions.

Alkali metal alkoxides (MOR) are produced by reactive distillation of alkali metal hydroxides (MOH) and alcohols (ROH) in a countercurrent distillation column, wherein the water of reaction formed according to the following reaction <1> is removed with the distillate. MOH+ROH

MOR+H₂O.

Such a process principle by which aqueous alkali metal hydroxide solution and gaseous methanol are run in countercurrent in a rectification column is disclosed for example in U.S. Pat. No. 2,877,274 A. This process is described again in generally unchanged form in WO 01/42178 A1.

Similar processes, which additionally employ an entraining agent such as for example benzene, are disclosed in GB 377,631 A and U.S. Pat. No. 1,910,331 A. This entraining agent is used to separate water and the water-soluble alcohol. In both patents the condensate is subjected to a phase separation to separate off the water of reaction.

Correspondingly, DE 96 89 03 C describes a process for continuous production of alkali metal alkoxides in a reaction column, wherein the water-alcohol mixture withdrawn at the top of the column is condensed and then subjected to a phase separation. The aqueous phase is discarded and the alcoholic phase is returned to the top of the column together with the fresh alcohol. EP 0 299 577 A2 describes a similar process, wherein the water in the condensate is separated off with the aid of a membrane.

The most industrially important alkali metal alkoxides are those of sodium and potassium, especially the methoxides and ethoxides. Their synthesis is frequently described in the prior art, for example in EP 1 997 794 A1.

The syntheses or alkali metal alkoxides by reactive rectification described in the prior art typically afford vapours comprising the employed alcohol and water. It is advantageous for economic reasons to reuse the alcohol comprised in the vapours as a reactant in the reactive distillation. The vapours are therefore typically supplied to a rectification column and the alcohol present therein is separated off (described for example in GB 737 453 A and U.S. Pat. No. 4,586,947 A). The thus recovered alcohol is then supplied to the reactive distillation as a reactant for example. Alternatively or in addition a portion of the alcohol vapour may be utilized for heating the rectification column (described in WO 2010/097318 A1). However, this requires that the vapour be compressed in order to achieve the temperature level required for heating the rectification column. The vapour is cooled between the compression stages, wherein a multistage compression is thermodynamically advantageous and an Intermediate cooling ensures that the maximum allowable temperature of the compressor is not exceeded.

Heat integration within the rectification stage for efficient utilization of employed energy is described in a different context in Ott, J., Gronemann, V., Pontzen, F., Fiedler, E., Grossmann, G., Kersebohm, D. B., Weiss, G. and Witte, C. (2012). Methanol. In Ullmann's Encyclopedia of Industrial Chemistry, (Ed.). (doi:10.1002/14358007.a16_485.pub3). Paragraph 5.4 of this citation discloses the workup of crude methanol obtained in conventional synthesis processes by rectification using a plurality of rectification columns. It generally proposes utilizing the heat of condensation of the vapour obtained at the rectification column at relatively high pressure for heating the rectification column at relatively low pressure. However, this citation discloses nothing about advantageous energy integration in the separation of water-methanol vapours produced in the reactive rectification of alkali metal alkoxides.

In the production of alkali metal alkoxides it is possible on an industrial scale, particularly in integrated plants (chemistry parks, technology parks), to utilize heating steam as the energy source for covering the energy demand. Said steam is typically generated in excess in Integrated plants and may be utilized.

However, depending on infrastructure and available energy sources heating steam is not always available and in certain cases electricity is more cost-effective. In these cases there is a need for processes for producing alkali metal alkoxides where a lowest possible proportion of energy need be covered through heating steam and a highest possible proportion may be covered through electricity.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide an improved process for production of alkoxides of sodium and potassium by reactive distillation. Said process shall especially allow energy-efficient utilization of the heat liberated during compression and cooling of the vapours. It should also cover a highest possible proportion of the energy requirements through electricity as an external energy source and feature a lowest possible heating steam demand.

The present invention accordingly provides a process for producing at least one alkali metal alkoxide of formula M_(A)OR, wherein R is a C₁ to C₆ hydrocarbon radical, preferably methyl or ethyl, and wherein M_(A) is selected from sodium, potassium, and wherein M_(A) is preferably sodium, wherein:

(a1) a reactant stream S_(AE1) comprising ROH is reacted with a reactant stream S_(AE2) comprising M_(A)OH in countercurrent at a pressure p_(3A) and a temperature T_(3A) in a reactive rectification column RR_(A) to afford a crude product RP_(A) comprising M_(A)OR, water, ROH, M_(A)OH, wherein a bottoms product stream S_(AP) comprising ROH and M_(A)OR is withdrawn at the lower end of RR_(A) and a vapor stream S_(AB) comprising water and ROH is withdrawn at the upper end of RR_(A), (a2) and optionally, simultaneously with and spatially separate from step (a1), a reactant stream S_(BE1) comprising ROH is reacted with a reactant stream S_(BE2) comprising M_(B)OH in countercurrent at a pressure p_(3B) and a temperature T_(3B) in a reactive rectification column RR_(B) to afford a crude product RP_(B) comprising M_(B)OR, water, ROH, M_(B)OH, wherein M_(B) is selected from sodium, potassium, and wherein M_(B) is preferably potassium, wherein a bottoms product stream S_(BP) comprising ROH and M_(B) OR is withdrawn at the lower end of RR_(B) and a vapour stream S_(BB) comprising water and ROH is withdrawn at the upper end of RR_(B), (b) the vapour stream S_(AB) and if step (a2) is performed the vapour stream S_(BB) in admixture with S_(AB) or separately from S_(AB) is passed into a first rectification column RD₁, to obtain a mixture G_(RD1) comprising water and ROH in the first rectification column RD₁, (c) the mixture G_(RD1) is in the first rectification column RD₁ at a pressure p₁ and a temperature T₁ separated into an ROH-comprising vapour stream S_(RDB1) at the upper end of RD₁ and a bottoms stream S_(RDS1) comprising water and ROH at the lower end of RD₁, (d) the bottoms stream S_(RDS1) is completely or partially passed into a second rectification column RD₂, to obtain a mixture G_(RD2) comprising water and ROH in the second rectification column RD₂, (e) the mixture G_(RD2) is at a pressure p₂ and a temperature T₂ separated into an ROH-comprising vapour stream S_(RDB2) at the top of RD₂ and a bottoms stream S_(RDS2) comprising water and optionally ROH at the lower end of RD₂, characterized in that p₁>p₂, p₁>p_(3A) and in the cases in which step (a2) is performed p₁>p_(3B) and wherein preferably in addition p_(3A)>p₂ and in the cases in which step (a2) is performed preferably in addition furthermore p_(3B)>p₂ and in that (f) energy from S_(RDB1) is transferred to the mixture G_(RD2) in the second rectification column RD₂.

The invention also includes the following embodiments:

1. Process for producing at least one alkali metal alkoxide of formula M_(A)OR, wherein R is a C₁ to C₆ hydrocarbon radical and wherein M_(A) is selected from sodium, potassium, wherein:

(a1) a reactant stream S_(AE1) comprising ROH is reacted with a reactant stream S_(AE2) comprising M_(A)OH in countercurrent at a pressure p_(3A) and a temperature T_(3A) in a reactive rectification column RR_(A) to afford a crude product RP_(A) comprising M_(A)OR, water, ROH, M_(A)OH, wherein a bottoms product stream S_(AP) comprising ROH and M_(A)OR is withdrawn at the lower end of RR_(A) and a vapour stream S_(AB) comprising water and ROH is withdrawn at the upper end of RR_(A), (a2) and optionally, simultaneously with and spatially separate from step (a1), a reactant stream S_(BE1) comprising ROH is reacted with a reactant stream S_(BE2) comprising M_(B)OH in countercurrent at a pressure p_(3B) and a temperature T_(3B) in a reactive rectification column RR_(B) to afford a crude product RP_(B) comprising M_(B)OR, water, ROH, M_(B)OH, wherein M_(B) is selected from sodium, potassium, wherein a bottoms product stream S_(BP) comprising ROH and M_(B)OR is withdrawn at the lower end of RR_(B) and a vapour stream S_(BB) comprising water and ROH is withdrawn at the upper end of RR_(B), (b) the vapour stream S_(AB) and if step (a2) is performed the vapour stream S_(BB) in admixture with S_(AB) or separately from S_(AB) is passed into a first rectification column RD₁, to obtain a mixture G_(RD1) comprising water and ROH in the first rectification column RD₁, (c) the mixture G_(RD1) is in the first rectification column RD₁ at a pressure p₁ and a temperature T₁ separated into an ROH-comprising vapour stream S_(RDB1) at the upper end of RD₁ and a bottoms stream S_(RDS1) comprising water and ROH at the lower end of RD₁, (d) the bottoms stream S_(RDS1) is completely or partially passed into a second rectification column RD₂, to obtain a mixture G_(RD2) comprising water and ROH in the second rectification column RD₂, (e) the mixture G_(RD2) is at a pressure p₂ and a temperature T₂ separated into an ROH-comprising vapour stream S_(RDB2) at the top of RD₂ and a bottoms stream S_(RDS2) comprising water at the lower end of RD₂, characterized in that p₁>p₂, p₁>p_(3A) and in the cases where step (a2) is performed p₁>p_(3B), and in that (f) energy from S_(RDB1) is transferred to the mixture G_(RD2) in the second rectification column RD₂. 2. Process according to embodiment 1, wherein in step (f) energy is directly transferred from S_(RDB1) to G_(RD2). 3. Process according to embodiment 2, wherein at least one of the steps (α-i), (α-ii), (α-iii) is performed: (α-i) energy from S_(RDB1) is transferred to a portion S_(RDS22) of the bottoms stream S_(RDS2) discharged from RD₂ and S_(RDS22) is then recycled into RD₂; (α-ii) at least one stream S_(RDX2) distinct from S_(RDB2) and S_(RDS2) comprising ROH and water is discharged from RD₂, energy is then transferred from S_(RDB1) to S_(RDX2) and S_(RDX2) is recycled into RD₂; (α-iii) S_(RDB1) is passed through RD₂, thus transferring energy from S_(RDB1) to G_(RD2). 4. Process according to embodiment 1, wherein in step (f) energy is indirectly transferred from S_(RDB1) to G_(RD2). 5. Process according to embodiment 4, wherein at least one of the steps (β-i), (β-ii), (β-iii) is performed: (β-i) a portion S_(RDS22) of the bottoms stream S_(RDS2) discharged from RD₂ is recycled into the second rectification column RD₂, wherein energy is transferred from S_(RDB1) to at least one heat transfer medium W_(i1) distinct from S_(RDS22) and then transferred from the at least one heat transfer medium W_(i1) to S_(RDS22) and S_(RDS22) is then recycled into RD₂; (β-ii) at least one stream S_(RDX2) distinct from S_(RDB2) and S_(RDS2) comprising ROH and water is discharged from RD₂ and energy is transferred from S_(RDB1) to at least one heat transfer medium W_(ii1) distinct from S_(RDX2) and then transferred from the at least one heat transfer medium W_(ii1) to S_(RDX2) and S_(RDX2) is then recycled into RD₂; (β-iii) energy is transferred from S_(RDB1) to at least one heat transfer medium W_(iii1) distinct from G_(RD2) and the at least one heat transfer medium W_(iii1) is then passed through RD₂, thus transferring energy from the at least one heat transfer medium W_(iii1) to G_(RD2). 6. Process according to embodiment 5, wherein each of W_(i1), W_(ii1), W_(iii1) is water. 7. Process according to any of embodiments 3, 5 and 6, wherein S_(RDX2) is withdrawn below the vapour stream S_(RDB2) on RD₂. 8. Process according to any of embodiments 1 to 7, wherein S_(RDB2) is at least partially employed as reactant stream S_(AE1) in the reactive rectification column RR_(A) and if step (a2) is performed alternatively or in addition employed as reactant stream S_(BE1) in the reactive rectification column RR_(B). 9. Process according to any of embodiments 1 to 8, wherein S_(RDB1) is at least partially employed as reactant stream S_(AE1) in the reactive rectification column RR_(A) and if step (a2) is performed alternatively or in addition employed as reactant stream S_(BE1) in the reactive rectification column RR_(B). 10. Process according to any of embodiments 1 to 9, wherein a stream S_(XE1) distinct from S_(AE1) and S_(BE1) comprising ROH is added to at least one of the columns selected from rectification column RD₁, rectification column RD₂, reactive rectification column RR_(A) and if step (a2) is performed is alternatively or in addition added to reactive rectification column RR_(B). 11. Process according to any of embodiments 1 to 10, wherein R is methyl or ethyl. 12. Process according to any of embodiments 1 to 11, wherein step (a2) is performed. 13. Process according to any of embodiments 1 to 12, wherein p_(3A)>p₂ and in addition in cases where step (a2) is performed p_(3B)>p₂. 14. Process according to any of embodiments 1 to 13, wherein the bottoms stream S_(RDS2) comprises water and ROH. 15. Process according to any of embodiments 1 to 14 which is carried out continuously.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a process according to the invention for producing alkali metal alkoxides with a corresponding interconnection of the rectification columns.

FIG. 2 shows a further process according to the invention for producing alkali metal alkoxides.

FIG. 3 shows one embodiment of a process not according to the invention for producing alkali metal alkoxides with a corresponding interconnection of the reactive rectification and rectification columns.

FIG. 4 shows a further embodiment of a process not according to the invention for producing alkali metal alkoxides with a corresponding interconnection of the rectification columns.

DETAILED DESCRIPTION OF THE INVENTION

4.1 Step (a1) of the Process According to the Invention

In step (a1) of the process according to the invention for producing at least one alkali metal alkoxide of formula M_(A)OR a reactant stream S_(AE1) comprising ROH is reacted with a reactant stream S_(AE2) comprising M_(A)OH in countercurrent at a pressure p_(3A) and a temperature T_(3A) in a reactive rectification column RR_(A) to afford a crude product RP_(A) comprising M_(A)OR, water, ROH, M_(A)OH.

According to the invention, a “reactive rectification column” is a rectification column in which the reaction according to step (a1) or step (a2) of the process of the invention proceeds at least in some parts. It may also be abbreviated to “reaction column”.

In step (a1) of the process according to the invention a bottoms product stream S_(AP) comprising ROH and M_(A)OR is withdrawn at the lower end of RR_(A). A vapour stream SAO comprising water and ROH is withdrawn at the upper end of RR_(A).

“Vapour stream” means that the respective stream is a gaseous stream.

In the process according to the invention, R is a C₁-C₆ hydrocarbon radical, preferably selected from the group consisting of methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl, isomers of pentyl such as n-pentyl, more preferably selected from the group consisting of methyl, ethyl, n-propyl, iso-propyl, yet more preferably selected from the group consisting of methyl, ethyl. R is particularly preferably methyl and ROH is accordingly methanol.

M_(A) is selected from sodium, potassium, preferably sodium.

The reactant stream S_(AE1) comprises ROH. In a preferred embodiment the mass fraction of ROH in S_(AE1) based on the total mass of the reactant stream S_(AE1) is ≥95% by weight, yet more preferably ≥99% by weight, wherein Sm otherwise comprises especially water.

The alcohol ROH used as reactant stream S_(AE1) in step (a1) of the process of the invention can also be a commercially available alcohol having a mass fraction of alcohol, based on the total mass of the reactant stream S_(AE1), of more than 99.8% by weight and a mass fraction of water, based on the total mass of the reactant stream S_(AE1), of up to 0.2% by weight.

The reactant stream S_(AE1) is preferably introduced in vapour form.

The reactant stream S_(AE2) comprises M_(A)OH. In a preferred embodiment S_(AE2) comprises not only M_(A)OH but also at least one further compound selected from water, ROH. S_(AE2) more preferably comprises water in addition to M_(A)OH, in which case S_(AE2) is then an aqueous solution of M_(A)OH.

When the reactant stream S_(AE2) comprises M_(A)OH and water the mass fraction of M_(A)OH based on the total weight of the reactant stream S_(AE2) is especially in the range from 10% to 55% by weight, preferably from 15% to 54% by weight, more preferably from 30% to 53% by weight and particularly preferably from 45% to 52% by weight, most preferably 50% by weight.

When the reactant stream S_(AE2) comprises M_(A)OH and ROH the mass fraction of M_(A)OH based on the total weight of the reactant stream S_(AE2) is especially in the range from 10% to 55% by weight, preferably from 15% to 54% by weight, more preferably from 30% to 53% by weight and particularly preferably from 45% to 52% by weight.

In the particular case in which the reactant stream S_(AE2) comprises both water and ROH in addition to M_(A)OH it is particularly preferable when the mass fraction of M_(A)OH based on the total weight of the reactant stream S_(AE2) is especially in the range from 10% to 55% by weight, preferably from 15% to 54% by weight, more preferably from 30% to 53% by weight and particularly preferably from 45% to 52% by weight.

Step (a1) of the process according to the invention is performed in a reactive rectification column (or “reaction column”) RR_(A).

Step (a2) of the process according to the Invention is performed in a reactive rectification column (or “reaction column”) RR_(B).

The reaction column RR_(A)/RR_(B) preferably contains internals. Suitable internals are, for example, trays, structured packings or unstructured packings. When the reaction column RR_(A)/RR_(B) contains trays, then bubble cap trays, valve trays, tunnel trays, Thormann trays, cross-slit bubble cap trays or sieve trays are suitable. When the reaction column RR_(A)/RR_(B) contains trays it is preferable to choose trays where not more than 5% by weight, more preferably less than 1% by weight, of the liquid trickles through the respective trays. The constructional measures required to minimize trickle-through of the liquid are familiar to those skilled in the art. In the case of valve trays, particularly tightly closing valve designs are selected for example. Reducing the number of valves also makes it possible to increase the vapour velocity in the tray openings to twice the value typically established. When using sieve trays it is particularly advantageous to reduce the diameter of the tray openings while maintaining or even increasing the number of openings.

When using structured or unstructured packings, structured packings are preferred in terms of uniform distribution of the liquid. In this embodiment it is further preferable when in all parts of the column cross section corresponding to more than 2% of the total column cross section the average ratio of liquid stream to vapour stream is not exceeded by more than 15%, more preferably by more than 3%, in respect of the liquid. This minimized liquid amount makes it possible for the capillary effect at the wire meshes to eliminate local peaks of liquid trickling density.

For columns comprising unstructured packings, especially comprising random packings, and for columns comprising structured packings, the desired characteristics of the liquid distribution may be achieved when the liquid trickling density in the edge region of the column cross section adjacent to the column shell, corresponding to about 2% to 5% of the total column cross section, is reduced compared to the other cross-sectional regions by up to 100%, preferably by 5% to 15%. This can easily be achieved by, for example, targeted distributions of the drip points of the liquid distributors or the holes thereof.

The process according to the Invention may be carried out either continuously or discontinuously. It is preferably carried out continuously.

According to the invention “reaction of a reactant stream S_(AE1) comprising ROH with a reactant stream S_(AE2) comprising M_(A)OH in countercurrent” is achieved, in particular, as a result of the feed point for at least a portion of the reactant stream S_(AE1) comprising ROH in step (a1) being located on the reaction column RR_(A) below the feed point of the reactant stream S_(AE2) comprising M_(A)OH.

The reaction column RR_(A) preferably comprises at least 2, in particular 15 to 40, theoretical plates between the feed point of the reactant stream S_(AE1) and the feed point of the reactant stream S_(AE2).

The reaction column RR_(A) is preferably operated as a pure stripping column. Accordingly the reactant stream S_(AE1) comprising ROH is especially supplied in vaporous form in the lower region of the reaction column RR_(A). Step (a1) of the process according to the invention also comprises the case where a portion of the reactant stream S_(AE1) comprising ROH is added in vapour form below the feed point of the reactant stream S_(AE2) comprising M_(A)OH but nevertheless at the upper end or in the region of the upper end of the reaction column RR_(A). This makes it possible to reduce the dimensions of the lower region of the reaction column RR_(A). When a portion of the reactant stream S_(AE1) comprising ROH, in particular methanol, is added especially in vaporous form at the upper end or in the region of the upper end of the reaction column RR_(A) only a fraction of 10% to 70% by weight, preferably of 30% to 50% by weight, (in each case based on the total amount of the alcohol ROH employed in step (a1) as S_(AE1)) is employed at the lower end of the reaction column RR_(A) and the remaining fraction is added in vaporous form in a single stream or divided into a plurality of substreams preferably 1 to 10 theoretical trays, particularly preferably 1 to 3 theoretical trays, below the feed point of the reactant stream S_(AE2) comprising M_(A)OH.

In the reaction column RR_(A), the reactant stream S_(AE1) comprising ROH is then reacted with the reactant stream S_(AE2) comprising M_(A)OH according to the reaction <1> described hereinabove to afford M_(A)OR and H₂O, where these products are present in admixture with the reactants ROH and M_(A)OH since an equilibrium reaction is concerned. Accordingly a crude product RP_(A) which contains not only the products M_(A)OR and water but also ROH and M_(A)OH is obtained in the reaction column RR_(A) in step (a1) of the process according to the invention.

The bottom product stream S_(AP) comprising ROH and M_(A)OR is obtained and withdrawn at the lower end of RR_(A).

A water-containing alcohol stream, previously described as “vapour stream S_(AB) comprising water and ROH”, is withdrawn at the upper end of RR_(A), preferably at the column top of RR_(A).

The amount of the alcohol ROH comprised by the reactant stream S_(AE1) is preferably chosen such that said alcohol also serves as a solvent for the alkali metal alkoxide M_(A)OR obtained in the bottoms product stream S_(AP). The amount of the alcohol ROH in the reactant stream S_(AE1) is preferably chosen to achieve in the bottom of reaction column RR_(A) the desired concentration of the alkali metal alkoxide solution which is withdrawn as a bottoms product stream S_(AP) comprising ROH and M_(A)OR.

In a preferred embodiment of the process of the invention, especially in cases in which S_(AE2) comprises water in addition to M_(A)OH, the ratio of the total weight (masses; unit: kg) of alcohol ROH used as reactant stream S_(AE1) in step (a1) to the total weight (masses; unit: kg) of M_(A)OH used as reactant stream S_(AE2) in step (a1) is from 1:1 to 50:1, more preferably 5:1 to 48:1, yet more preferably 9:1 to 35:1, yet still more preferably 10:1 to 30:1, yet still more preferably 13:1 to 22:1, most preferably 14:1.

The reaction column RR_(A) is operated with or without, preferably without, reflux.

“Without reflux” is to be understood as meaning that the vapour stream S_(AB) withdrawn at the upper end of RR_(A) comprising water and ROH is completely supplied to the first rectification column RD₁ according to step (b). The vapour stream S_(AB) comprising water and ROH is preferably supplied to the first rectification column RD₁ in vaporous form.

“With reflux” is to be understood as meaning that the vapour stream S_(AB) withdrawn at the upper end of the respective column, reaction column RR_(A) in step (a1), comprising water and ROH is not completely discharged, i.e. is not completely supplied to the first rectification column RD₁ in step (b), but rather is at least partially, preferably partially, recycled to the respective column, reaction column RR_(A) in step (a1), as reflux. In the cases where such a reflux is established, the reflux ratio is preferably 0.05 to 0.99, more preferably 0.1 to 0.9, yet more preferably 0.11 to 0.34, particularly preferably 0.14 to 0.27 and very particularly preferably 0.17 to 0.24. A reflux may be established by attaching to the top of the respective column, reaction column RR_(A) in step (a1), a condenser K_(RRA) in which the vapour stream S_(AB) is at least partially condensed and sent back to the respective column, reaction column RR_(A) in step a1). Generally and in the context of the present invention a reflux ratio is to be understood as meaning the ratio of the mass flow (kg/h) recycled to the respective column in liquid form (reflux) to the mass flow (kg/h) discharged from the respective column in liquid form (distillate) or gaseous form (vapours).

In the embodiment in which a reflux is established on the reaction column RR_(A) the alcohol M_(A)OH employed in step (a1) as reactant stream S_(AE2) may also be at least partially, preferably partially, mixed with the reflux stream and the resulting mixture thus supplied to step (a1).

Step (a1) of the process according to the invention is in particular performed at a temperature T_(3A) in the range from 25° C. to 200° C., preferably in the range from 45° C. to 150° C., more preferably in the range from 47° C. to 120° C. more preferably in the range from 60° C. to 110° C.

Step (a1) of the process according to the invention is in particular performed at a pressure p_(3A) of 0.5 bar to 40 bar, preferably in the range from 0.75 bar to 5 bar, more preferably in the range from 1 bar to 2 bar, more preferably in the range from 1 bar to 1.8 bar, yet more preferably at 1.1 bar to 1.6 bar. It is an essential feature of the invention that when establishing the pressure p_(3A): p₁>p_(3A). It is especially also the case that p_(3A)>p₂.

In a preferred embodiment the reaction column RR_(A) comprises at least one evaporator which is in particular selected from intermediate evaporators VZ_(3A) and bottoms evaporators VS_(3A). The reaction column RR_(A) particularly preferably comprises at least one bottoms evaporator VS_(3A). Evaporators are special embodiments of heat exchangers WT.

Condensers K are likewise special embodiment of heat exchangers WT. Typical condensers are known to those skilled in the art. These are preferably employed as liquefiers at the top of rectification columns and reaction columns. In the direct energy transfer from the top stream of one column to the bottoms or intermediate stream of another column a condenser of one column may simultaneously be employed as an evaporator of the other column (as shown in the examples).

According to the invention “intermediate evaporators” VZ (for example VZ_(3A) in RR_(A), VZ_(3B) in RR_(B), VZ_(RD1) in RD₁, VZ_(RD2) in RD₂) are to be understood as meaning evaporators arranged above the bottom of the respective column, in particular above the bottom of the reaction column RR_(A)/RR_(B) or above the bottom of the rectification column RD₁ or RD₂. They in particular evaporate crude product RP_(A)/RP_(B) or S_(RDX1Z) as sidestream.

According to the invention “bottoms evaporators” VS (for example VS_(3A) at RR_(A), VS_(3B) at RR_(B), VS_(RD1) at RD₁, VS_(RD2) at RD₂) are to be understood as meaning evaporators which heat the bottom of the respective column. In particular the bottom of the reaction column RR_(A)/RR_(B) or rectification column RD₁ or RD₂. They evaporate bottoms product stream (for example S_(AP)/S_(BP) or S_(RDX1S)).

A vaporizer is typically arranged outside the respective reaction column or rectification column. The mixture to be vaporized in the vaporizer is taken off from the column via an offtake or “offtake point” and fed to the at least one vaporizer.

The evaporated mixture is recycled back into the respective column, optionally with a residual proportion of liquid, via a feed or “feed point”. When the evaporator is an intermediate evaporator the takeoff by means of which the respective mixture is withdrawn and supplied to the evaporator is a sidestream takeoff and the feed by means of which the evaporated respective mixture is sent back to the column is a sidestream feed. When the evaporator is a bottoms evaporator. i.e. heats the column bottom, at least a portion of the bottom takeoff stream is evaporated and recycled into the respective column in the region of the bottom.

However, it is alternatively also possible for example on a suitable tray when using an intermediate evaporator or in the bottom of the respective column to provide tubes which are traversed by the relevant heating medium. In this case, the vaporization occurs on the tray or In the bottom region of the column. However, it is preferable to arrange the vaporizer outside the respective column.

Suitable evaporators employable as intermediate evaporators and bottoms evaporators include for example natural circulation evaporators, forced circulation evaporators, forced circulation evaporators with decompression, steam boilers, falling film evaporators or thin film evaporators.

Heat exchangers for the vaporizer typically employed in the case of natural circulation evaporators and forced circulation evaporators are shell and tube or plate apparatuses. When using a shell and tube exchanger the heating medium may either flow through the tubes with the mixture to be evaporated flowing around the tubes or else the heating medium may flow around the tubes with the mixture to be evaporated flowing through the tubes. In the case of a falling-film evaporator, the mixture to be vaporized is typically introduced as a thin film on the inside of a tube and the tube is heated externally. In contrast to a falling-film evaporator, a thin-film evaporator additionally comprises a rotor with wipers which distributes the liquid to be evaporated on the inner wall of the tube to form a thin film.

In addition to the recited evaporator types it is also possible to employ any desired further evaporator type known to those skilled in the art and suitable for use on a rectification column.

When the reaction column RR_(A)/reaction column RR_(B) comprises an intermediate evaporator VZ_(3A) or VZ_(3B) as it is preferable when the respective intermediate evaporator is arranged in the stripping region of the reaction column RR_(A) in the region of the feed point of the reactant stream S_(AE1) or in the case of the reaction column RR_(B) in the region of the feed point for the reactant stream S_(BE1). This makes it possible to introduce a predominant portion of the heat energy via the intermediate evaporator VZ_(3A)/VZ_(3B). It is thus possible for example to introduce more than 80% of the energy via the intermediate vaporizer. According to the Invention the intermediate evaporator VZ_(3A)/VZ_(3B) is preferably arranged and/or configured such that it introduces more than 50%, in particular more than 75%, of the total energy required for the reactive rectification.

When the reaction column RR_(A)/reaction column RR_(B) has an intermediate evaporator VZ_(3A) or VZ_(3B) it is additionally advantageous when the intermediate evaporator is arranged such that the reaction column RR_(A)/RR_(B) has 1 to 50 theoretical trays below the intermediate evaporator and 1 to 200 theoretical trays above the intermediate evaporator. It is especially preferred when the reaction column RR_(A)/RR_(B) then has 2 to 10 theoretical trays below the intermediate evaporator and 20 to 50 theoretical trays above the intermediate evaporator.

When the reaction column RR_(A)/reaction column RR_(B) comprises an intermediate evaporator VZ_(3A)/VZ_(3B) it is also advantageous when the sidestream takeoff (i.e. the “takeoff point E_(RRA)” on the reaction column RR_(A)/the “takeoff point E_(RRB)” on the reaction column RR_(B)) by means of which the crude product RP_(A)/RP_(B) is supplied to the intermediate evaporator VZ_(3A)/VZ_(3B) and the sidestream feed (i.e. the “feed point Z_(RRA)” on the reaction column RR_(A)/the “feed point Z_(RRB)” on the reaction column RR_(B)) by means of which the evaporated crude product RP_(A)/RP_(B) from the intermediate evaporator VZ_(3A)/VZ_(3B) is sent back to the respective reaction column RR_(A)/RR_(B) are positioned between the same trays of the reaction column RR_(A)/reaction column RR_(B). However, it is also possible for the sidestream takeoff and sidestream feed to be arranged at different heights.

In a preferred embodiment when using an Intermediate evaporator VZ_(3A)/VZ_(3B) in RR_(A)/RR_(B) the diameter of the reaction column RR_(A)/RR_(B) above the intermediate evaporator RR_(A)/RR_(B) is greater than the diameter of the reaction column RR_(A)/RR_(B) below the intermediate evaporator VZ_(3A)/VZ_(3B). This has the advantage of allowing capital expenditure savings.

In such an intermediate evaporator VZ_(3A)/VZ_(3B) liquid crude product RP_(A) comprising M_(A)OR, water, ROH, M_(A)OH present in the reaction column RR_(A) or liquid crude product RP_(B) comprising M_(B)OR, water, ROH, M_(B)OH present in the reaction column RR_(B) may be converted into the gaseous state if already in the gaseous state heated further, thus improving the efficiency of the reaction of step (a1)/(a2) in the process according to the invention.

Arranging one or more intermediate evaporators VZ_(3A) in the upper region of the reaction column RR_(A) or one or more intermediate evaporators VZ_(3B) in the upper region of the reaction column RR_(B) makes it possible to reduce the dimensions in the lower region of the reaction column RR_(A)/RR_(B). In the embodiment having at least one, preferably two or more, intermediate evaporators VZ_(3A)/VZ_(3B) it is also possible to supply substreams of the ROH in liquid form in the upper region of the reaction column RR_(A)/RR_(B).

According to the invention bottoms evaporators are arranged at the bottom of the reaction column RR_(A)/RR_(B) and are then referred to as “VS_(3A)” and “VS_(3B)”. Bottoms product stream S_(AP)/S_(BP) present in the reaction column RR_(A)/RR_(B) may be passed into such a bottoms evaporator and ROH at least partially removed therefrom to obtain a bottoms product stream S_(AP*) having an elevated mass fraction of M_(A)OR compared to S_(AP)/to obtain a bottoms product stream S_(BP*) having an elevated mass fraction of M_(B)OR compared to S_(BP).

In step (a1) of the process according to the invention a bottoms product stream S_(AP) comprising ROH and M_(A)OR is withdrawn at the lower end of the reaction column RR_(A).

It is preferable when the reaction column RR_(A) comprises at least one bottoms evaporator VS_(3A) through which the bottoms product stream S_(AP) is then at least partially passed to at least partially remove ROH, thus affording a bottoms product stream S_(AP*) having an elevated mass fraction of M_(A)OR compared to S_(AP).

The mass fraction of M_(A)OR in the bottoms product stream S_(AP*) is especially elevated compared to the mass fraction of M_(A)OR in the bottoms product stream S_(AP) by at least 1%, preferably by ≥2%, more preferably by ≥5%, yet more preferably by ≥10%, yet still more preferably by ≥20%, yet still more preferably by ≥30%, yet still more preferably by ≥40%, yet still more preferably by ≥50%, yet still more preferably by ≥100%, yet still more preferably by ≥150%.

It is preferable when S_(AP) or, if at least one bottoms evaporator VS_(3A) is used, through which the bottoms product stream S_(AP) is at least partially passed to at least partially remove ROH, S_(AP*) has a mass fraction of M_(A)OR in ROH in the range from 1% to 50% by weight, preferably 5% to 32% by weight, more preferably 15% to 32% by weight, most preferably 30% to 32% by weight, in each case based on the total mass of S_(AP)/S_(AP*).

The mass fraction of residual water in S_(AP)/S_(AP*) is preferably <1% by weight, preferably <0.1% by weight, more preferably <0.01% by weight, based on the total mass of S_(AP)/S_(AP*).

The mass fraction of reactant M_(A)OH in S_(AP)/S_(AP*) is preferably <1% by weight, preferably <0.1% by weight, more preferably <0.01% by weight, based on the total mass or S_(AP)/S_(AP*).

4.2 Step (a2) of the Process According to the Invention (Optional)

According to the invention step (a2) is performed or not performed. In the optional step (a2), which proceeds simultaneously with and spatially separately from step (a1) of the process according to the Invention, a reactant stream S_(BE1) comprising ROH is reacted with a reactant stream S_(BE2) comprising M_(B)OH in countercurrent at a pressure pau and a temperature T_(3B) in a reactive rectification column RR_(B) to afford a crude product mixture RP_(B) comprising M_(B)OR, water, ROH, M_(B)OH.

In the optional step (a2) of the process according to the Invention a bottoms product stream S_(BP) comprising ROH and M_(B)OR is withdrawn at the lower end of RR_(B). A vapour stream S_(BB) comprising water and ROH is withdrawn at the top end of RR_(B).

M_(B) is selected from sodium, potassium, and preferably potassium.

The reactant stream S_(BE1) comprises ROH. In a preferred embodiment the mass fraction of ROH in S_(BE1) based on the total mass of the reactant stream S_(BE1) is a ≥95% by weight, yet more preferably ≥99% by weight, wherein S_(BE1) otherwise comprises especially water.

The alcohol ROH used as reactant stream S_(BE1) in the optional step (a2) of the process of the Invention can also be a commercial alcohol having a mass fraction of alcohol, based on the total mass of the reactant stream S_(BE1), of more than 99.8% by weight and a proportion by mass of water, based on the total mass of the reactant stream S_(BE1), of up to 0.2% by weight.

The reactant stream S_(BE1) is preferably introduced in vapour form.

The reactant stream S_(BE2) comprises M_(B)OH. In a preferred embodiment S_(BE2) comprises not only M_(B)OH but also at least one further compound selected from water, ROH. It is yet more preferable when S_(BE2) comprises water in addition to M_(B)OH, thus rendering S_(BE2) an aqueous solution of M_(B)OH.

When the reactant stream S_(BE2) comprises M_(B)OH and water the mass fraction of M_(B)OH based on the total weight of the reactant stream S_(BE2) is especially in the range from 10% to 55% by weight, preferably from 15% to 54% by weight, more preferably from 30% to 53% by weight and particularly preferably from 45% to 52% by weight, most preferably 50% by weight.

When the reactant stream S_(BE2) comprises M_(B)OH and ROH the mass fraction of M_(B)OH based on the total weight of the reactant stream S_(BE2) is especially in the range from 10% to 55% by weight, preferably from 15% to 54% by weight, more preferably from 30% to 53% by weight and particularly preferably from 45% to 52% by weight.

In the particular case in which the reactant stream S_(BE2) comprises both water and ROH in addition to M_(B)OH it is particularly preferable when the mass fraction of M_(B)OH based on the total weight of the reactant stream S_(BE2) is especially in the range from 10% to 55% by weight, preferably from 15% to 54% by weight, more preferably from 30% to 53% by weight and particularly preferably from 45% to 52% by weight.

Step (a2) of the process according to the invention is performed in a reactive rectification column (or “reaction column”) RR_(B). Preferred embodiments of the reaction column RR_(B) are described in section 4.1.

According to the invention “reaction of a reactant stream S_(BE1) comprising ROH with a reactant stream S_(BE2) comprising M_(B)OH in countercurrent” is especially achieved as a result of the feed point for at least a portion of the reactant stream S_(BE1) comprising ROH in optional step (a2) being arranged below the feed point for the reactant stream S_(BE2) comprising M_(B)OH on the reaction column RR_(B).

The reaction column RR_(B) preferably comprises at least 2, in particular 15 to 40, theoretical trays between the feed point of the reactant stream S_(BE1) and the feed point of the reactant stream S_(BE2).

The reaction column RR_(B) is preferably operated as a pure stripping column. Accordingly the reactant stream S_(BE1) comprising ROH is especially supplied in vaporous form in the lower region of the reaction column RR_(B). The optional step (a2) of the process of the Invention also encompasses the case of part of the reactant stream S_(BE1) comprising ROH being introduced in vapour form below the feed point of the reactant stream S_(BE2) comprising alkaline solution M_(B)OH but at the upper end or in the region of the upper end of the reaction column RR_(B). This makes it possible to reduce the dimensions of the lower region of the reaction column RR_(B). When a portion of the reactant stream S_(BE1) comprising ROH, in particular methanol, is added especially in vaporous form at the upper end or in the region of the upper end of the reaction column RR_(B) only a fraction of in particular 10% to 70% by weight, preferably of 30% to 50% by weight, (in each case based on the total amount of the alcohol ROH employed in optional step (a2)) is employed at the lower end of the reaction column RR_(B) and the remaining fraction is added in vaporous form in a single stream or divided into a plurality of substreams preferably 1 to 10 theoretical trays, particularly preferably 1 to 3 theoretical trays, below the feed point of the reactant stream S_(BE2) comprising M_(B)OH.

In the reaction column RR_(B) the reactant stream S_(BE1) comprising ROH is then reacted with the reactant stream S_(BE1) comprising M_(B)OH according to the reaction <1> described hereinabove to afford M_(B)OR and H₂O, where these products are present in admixture with the reactants ROH and M_(B)OH since an equilibrium reaction is concerned. Accordingly a crude product RP_(B) which contains not only the products M_(B)OR and water but also ROH and M_(B)OH s obtained in the reaction column RR_(B) in optional step (a2) of the process according to the invention.

The bottom product stream S_(BP) comprising ROH and M_(B) OR is obtained and withdrawn at the lower end of RR_(B).

A water-containing alcohol stream, previously described as “vapor stream S_(BB) comprising water and ROH”, is withdrawn at the upper end of RR_(B), preferably at the top or RR_(B).

This vapour stream S_(BB) comprising water and ROH is supplied to step (b) of the process according to the invention. Said stream is mixed with S_(AB) before being supplied to step (b) of the process according to the invention or is not, i.e. is supplied to step (b) of the process according to the invention separately from S_(AB). Vapour stream S_(BB) is preferably mixed with S_(AB) and the resulting mixed vapour stream is then introduced into step (b) of the process of the invention.

The amount of alcohol ROH present in the reactant stream S_(BE1) is preferably selected so that it simultaneously serves as solvent for the alkali metal alkoxide M_(B)OR present in the bottom product stream S_(BP). The amount or the alcohol ROH in the reactant stream S_(BE1) is preferably chosen to achieve in the bottom of the reaction column the desired concentration of the alkali metal alkoxide solution which is withdrawn as a bottoms product stream S_(BP) comprising ROH and M_(B)OR.

In a preferred embodiment of optional step (a2) of the process according to the Invention, and especially in the cases where S_(BE2) comprises water in addition to M_(B)OH, the ratio of the total weight (masses; units: kg) of alcohol employed in step (a2) as reactant stream S_(BE1) ROH to the total weight (masses; unit: kg) of M_(B)OH employed in step (a2) as reactant stream S_(BE2) is 1:1 to 50:1, more preferably 5:1 to 48:1, yet more preferably 9:1 to 35:1, yet still more preferably 10:1 to 30:1, yet still more preferably 13:1 to 22:1, most preferably 14:1.

The reaction column RR_(B) is operated with or without, preferably without, reflux.

“Without reflux” is to be understood as meaning that the vapour stream S_(BB) withdrawn at the upper end of RR_(B) comprising water and ROH is completely supplied to the rectification column RD₁ according to step (b). The vapour stream S_(BB) comprising water and ROH is preferably supplied to the rectification column RD₁ in vaporous form.

“With reflux” is to be understood as meaning that the vapor stream S_(BB) withdrawn at the upper end of the respective column, reaction column RR_(B) in step (a2), comprising water and ROH is not completely discharged, i.e. is not completely supplied to the first rectification column RD₁ in step (b), but rather is at least partially, preferably partially, recycled to the respective column, reaction column RR_(B) in step (a2), as reflux. In the cases where such a reflux is established the reflux ratio is preferably 0.05 to 0.99, more preferably 0.1 to 0.9, yet more preferably 0.11 to 0.34, particularly preferably 0.14 to 0.27 and very particularly preferably 0.17 to 024. A reflux may be established by attaching at the top of the respective column, reaction column RR_(B) in step (a2), a condenser K_(RRB) in which the vapour stream S_(BB) is at least partially condensed and sent back to the respective column, reaction column RR_(B) in step (a2).

In the embodiment in which a reflux is established on the reaction column RR_(B) the alcohol M_(B)OH employed in optional step (a2) as reactant stream S_(BE2) may also be at least partially, preferably partially, mixed with the reflux stream and the resulting mixture thus supplied to step (a2).

Optional step (a2) of the process according to the invention is in particular performed at a temperature T_(3B) in the range from 25° C. to 200° C., preferably in the range from 45° C. to 150° C., more preferably in the range from 47° C. to 120° C., more preferably in the range from 60° C. to 110° C.

Optional step (a2) of the process according to the invention is in particular performed at a pressure p_(3B) of 0.5 bar to 40 bar, preferably in the range from 0.75 bar to 5 bar, more preferably in the range from 1 bar to 2 bar, more preferably in the range from 1 bar to 1.8 bar, yet more preferably at 1.1 bar to 1.8 bar. It is an essential feature of the invention that when establishing the pressure p_(3B): p₁>p_(3B). It is especially also the case that p_(3B)>p₂.

In a preferred embodiment the reaction column RR_(B) comprises at least one evaporator which is in particular selected from intermediate evaporators V_(AB) and bottoms evaporators V_(SB). The reaction column RR_(B) particularly preferably comprises at least one bottoms evaporator VS_(3B).

In optional step (a2) of the process according to the invention a bottoms product stream S_(BP) comprising ROH and M_(B)OR is withdrawn at the lower end of the reaction column RR_(B).

It is preferable when the reaction column RR_(B) comprises at least one bottoms evaporator VS_(3B) through which the bottoms product stream S_(BP) is then at least partially passed to at least partially remove ROH, thus affording a bottoms product stream S_(BP*) having an elevated mass fraction of M_(B)OR compared to S_(BP).

The mass fraction of M_(B) OR in the bottoms product stream S_(BP*) is especially elevated compared to the mass fraction of M_(B) OR in the bottoms product stream S_(BP) by at least 1%, preferably by ≥2%, more preferably by ≥5%, yet more preferably by ≥10%, yet still more preferably by ≥20%, yet still more preferably by ≥30%, yet still more preferably by ≥40%, yet still more preferably by a ≥50%, yet still more preferably by ≥100%, yet still more preferably by ≥150%.

It is preferable when S_(BP) or, if at least one bottoms evaporator VS_(3B) is used, through which the bottoms product stream S_(BP) is at least partially passed to at least partially remove ROH, S_(BP*) has a mass fraction of M_(B) OR in ROH in the range from 1% to 50% by weight, preferably 5% to 32% by weight, more preferably 10% to 32% by weight, most preferably 15% to 30% by weight, in each case based on the total mass of S_(BP)/S_(BP*).

The mass fraction of residual water in S_(BP)/S_(BP*) is preferably <1% by weight, preferably <0.1% by weight, more preferably <0.01% by weight, based on the total mass of S_(BP)/S_(BP*).

The mass fraction of reactant M_(B)OH in S_(BP)/S_(BP*) is preferably <1% by weight, preferably <0.1% by weight, more preferably <0.01% by weight, based on the total mass of S_(BP)/S_(BP*).

In the embodiments of the present process in which step (a2) is also performed it is preferable when the bottoms product stream S_(AP) is at least partially passed through a bottoms evaporator VS_(3A) and ROH is at least partially removed from SA to afford a bottoms product stream S_(AP*) having an elevated mass fraction of M_(A)OR compared to S_(AP) and/or, preferably and, the bottoms product stream S_(BP) is at least partially passed through a bottoms evaporator VS_(3B) and ROH is at least partially removed from S_(BP) to afford a bottoms product stream S_(BP*) having an elevated mass fraction of M_(B)OR compared to S_(BP).

In the embodiments of the present invention in which it is performed step (a2) of the process according to the invention is performed simultaneously with and spatially separate from step (a1). Spatial separation is ensured by performing steps (a1) and (a2) in the two reaction columns RR_(A) and RR_(B).

In an advantageous embodiment of the invention the reaction columns RR_(A) and RR_(B) are accommodated in one column shell, where the column is at least partially subdivided by at least one dividing wall. Such a column having at least one dividing wall will according to the invention be referred to as “DWC”. Such dividing wall columns are familiar to those skilled in the art and are described for example in U.S. Pat. No. 2,295,258, EP 0 122 387 A2, EP 0 128 288 A2, WO 2010/097318 A1 and I. Dejanović, Lj. Matijas̆ević, Z̆. Olujić, Chemical Engineering and Processing 2010, 49, 559-580. In the dividing wall columns suitable for the process according to the invention the dividing walls preferably extend to the floor and, in particular, preferably span at least a quarter, more preferably at least a third, yet more preferably at least half, yet more preferably at least two thirds, yet still more preferably at least three quarters, of the column by height They divide the columns into at least two reaction spaces in which spatially separate reactions may be carried out. The reaction spaces provided by the at least one dividing wall may be of identical or different sizes.

In this embodiment the bottoms product streams S_(AP) and S_(BP) may be separately withdrawn in the respective regions separated by the dividing wall and preferably passed through the bottoms evaporator VS_(3A)/VS_(3B) attached for each reaction space formed by the at least one reaction wall in which ROH is at least partially removed from S_(AP)/S_(BP) to afford S_(AP*)/S_(BP*).

4.3 Step (b) of the Process According to the Invention

In step (b) of the process according to the invention the vapour stream S_(AB) and if step (a2) is performed the vapour stream S_(BB) in admixture with S_(AB) or separately from S_(AB) is passed into a first rectification column RD₁,

to obtain a mixture G_(RD1) comprising water and ROH in the rectification column RD₁.

In the optional embodiment of the process according to the invention in which step (a2) is performed the vapour stream S_(BB) is preferably mixed with S_(AB) and the obtained mixed vapour S_(ABB) then introduced into a rectification column RD₁.

In one embodiment of the present invention (when p_(3A)<p₁/p_(3B)<p₁) the vapour stream S_(AB) and, in cases where the optional step (a2) is performed, the vapour stream S_(BB) may be compressed before they are passed into the rectification column RD₁. This may be effected via a compressor VD₃₁. However, in the embodiments of the present invention in which p_(3A)>p₁ and p_(3B)>p₁ the provision of a compressor VD₃₁ is not necessary and it is therefore possible to save on the provision thereof and the electrical energy required therefor.

It will be appreciated that even in the embodiments in which the optional step (a2) is performed and S_(BB) is introduced into the rectification column RD₁ separately from S_(AB) S_(AB) and S_(BB) undergo mixing in the rectification column RD₁ with the result that a mixture G_(RD1) comprising water and ROH is always obtained in the first rectification column RD₁ after performance of step (b).

Any desired rectification column known to those skilled in the art may be employed as rectification column RD₁ in step (b) of the process according to the invention. The rectification column RD₁ preferably contains internals. Suitable internals are, for example, trays, unstructured packings or structured packings. As trays, use is normally made of bubble cap trays, sieve trays, valve trays, tunnel trays or slit trays. Unstructured packings are generally beds of random packing elements. Packing elements normally used are Raschig rings, Pall rings, Berl saddles or Intalox® saddles. Structured packings are for example marketed under the trade name Mellapack® from Sulzer. Apart from the internals mentioned, further suitable internals are known to a person skilled in the art and can likewise be used.

Preferred internals have a low specific pressure drop per theoretical plate. Structured packings and random packing elements have, for example, a significantly lower pressure drop per theoretical plate than trays. This has the advantage that the pressure drop in the rectification column remains as low as possible and the mechanical power of the compressor and the temperature of the alcohol/water mixture G_(RD1) to be evaporated thus remains low.

When the rectification column RD₁ contains structured packings or unstructured packings these may be divided or in the form of an uninterrupted packing. However, typically at least two packings are provided, one packing above the feed point of the vapour stream S_(AB)/the feed points of the two vapour streams S_(AB) and S_(BB) and a packing below the feed point of the vapour stream S_(AB)/the feed points of the two vapour streams S_(AB) and S_(BB)/the feed point of the mixed vapours S_(ABB). If an unstructured packing is used, for example a random packing, the random packing elements are typically disposed on a suitable sieve tray or mesh tray.

At the end or step (b) of the process according to the invention a mixture G_(RD1) comprising water and ROH is finally obtained in the rectification column RD₁. The composition of the mixture G_(RD1) results in particular from the composition of the vapour stream S_(AB) in particular and if step (a2) is performed partly from the composition of the two vapour streams S_(AB) and S_(BB) in particular.

4.4 Step (c) of the Process According to the Invention

In step (c) of the process according to the invention the mixture G_(RD1) comprising water and ROH is in the first rectification column RD₁ at a pressure p₁ and a temperature T₁ separated into an ROH-comprising vapour stream S_(RDB1) at the upper end (=top) of RD₁ and a bottoms stream S_(RDS1) comprising water and ROH at the lower end (=bottom) of RD₁.

With the exception of the proviso that p₁>p₂ the pressure p₁ in RD₁ may be chosen by those skilled in the art according to their knowledge of the art. It is preferably in the range between 1 bar and 20 bar, preferably 1 bar and 15 bar, more preferably 2 to 14 bar, yet more preferably 4.00 to 11.00 bar, yet more preferably 6.00 to 10.00 bar, yet more preferably 7.00 to 8.90 bar, wherein, simultaneously, p₁>p₂.

The temperature T₁ in RD₁ may be chosen by those skilled in the art according to their knowledge of the art. It is preferably in the range from 40° C. to 220° C., preferably from 60° C. to 190° C.

In a preferred embodiment p_(3A)>p₂ and in addition in cases where step (a2) is performed p_(3B)>p₂. As a result of this established pressure the total energy demand of the process is surprisingly minimized compared to the embodiments where p_(3A)<p₂/p_(3B)<p₂

The separation according to step (c) of the process according to the invention is a distillative separation of the alcohol/water mixture G_(RD1) as is known to those skilled in the art.

At the lower end (also: “bottom”) of the rectification column RD₁ a bottoms stream S_(RDS1) still comprising alcohol ROH is obtained. S_(RDS1) comprises ROH in a mass fraction of in particular 0.005% to 95% by weight, preferably 25% to 95% by weight, based on the total mass of S_(RDS1). S_(RDS1) preferably comprises essentially water in addition to the alcohol ROH.

In a preferred embodiment of the Invention S_(RDB1) is at least partially employed as reactant stream S_(AE1) in the reactive rectification column RR_(A) and if step (a2) is performed alternatively or in addition employed as reactant stream S_(BE1) in the reactive rectification column RR_(B).

Also obtained at the top of the rectification column RD₁ is the vapour stream S_(RDB1) comprising ROH. The preferred mass fraction of ROH in this vapour stream S_(RDB1) is ≥99% by weight, preferably ≥99.6% by weight, more preferably ≥99.9% by weight, in each case based on the total mass of S_(RDB1), wherein the remainder is especially water.

In step (c) the vapour S_(AB) or S_(AB) and S_(BB) obtained in step (a1) or step (a1) and (a2) is subjected to distillative separation. These vapours comprise essentially the alcohol ROH and water. In particular, S_(AB) or S_(AB) and S_(BB) are each a water/alcohol mixture in which the mass fraction of ROH is preferably in the range >80% by weight, more preferably >85% by weight, yet more preferably >90% by weight (based on the total mass of S_(AB) or S_(AB) and S_(BB)). Thus in particular G_(RD1) too is an alcohol/water mixture in which the mass fraction of ROH is preferably in the range >80% by weight, more preferably >95% by weight, yet more preferably >90% by weight (based on the total mass of G_(RD1)).

4.5 Step (d) of the Process According to the Invention

In step (d) of the process according to the invention the bottoms stream S_(RDS1) is completely or partially, preferably partially, passed into a second rectification column RD₂.

This affords a mixture G_(RD2) comprising water and ROH in the second rectification column RD₂.

In the embodiment of the present invention in which S_(RDS1) is partially passed into RD₂ this is especially performed such that a first portion S_(RDS11) of the bottoms stream S_(RDS1) discharged from the first rectification column RD₁ is passed into a second rectification column RD₂ and a second portion S_(RDS12) of the bottoms stream S_(RDS1) discharged from the first rectification column RD₁ is recycled into the first rectification column RD₁. It is yet more preferable when energy is transferred to S_(RDS12), yet still more preferable when S_(RDS12) is heated. Once S_(RDS12) has been recycled to RD₁ it undergoes mixing in RD₁ with G_(RD1) and thus provides energy for separating G_(RD1) according to step (c).

In this preferred embodiment of step (d) of the process according to the invention it is yet more preferable when the ratio of the masses (in kg) of S_(RDS11) to S_(RDS12) are in the range 9:1 to 1:9, yet more preferably 4:1 to 1:4, yet more preferably 7:3 to 3:7, yet more preferably 3:2 to 2:3, yet more preferably 1:1.

In this preferred embodiment of step (d) of the process according to the invention it is possible to supply energy to the stream S_(RDS12). In a preferred embodiment this is effected when the stream S_(RDS12) is passed through a bottoms evaporator VS_(RD1) in which energy is transferred from a heat transfer medium to S_(RDS12). This energy transfer may advantageously be undertaken when S_(RDS12) and the heat transfer medium are passed through a bottoms evaporator VS_(RD1). After the recycling of S_(RDS12) into the reaction column RR_(A) S_(RDS12) then transfers the energy to G_(RD1).

Any desired rectification column known to those skilled in the art may be employed as rectification column RD₂ in step (d) of the process according to the invention. The rectification column RD₂ preferably contains internals. Suitable internals are, for example, trays, unstructured packings or structured packings. As trays, use is normally made of bubble cap trays, sieve trays, valve trays, tunnel trays or silt trays. Unstructured packings are generally beds of random packing elements. Packing elements normally used are Raschig rings, Pall rings, Berl saddles or Intalox® saddles. Structured packings are for example marketed under the trade name Mellapack® from Sulzer. Apart from the internals mentioned, further suitable internals are known to a person skilled in the art and can likewise be used.

Preferred internals have a low specific pressure drop per theoretical plate. Structured packings and random packing elements have, for example, a significantly lower pressure drop per theoretical plate than trays. This has the advantage that the pressure drop in the rectification column RD₂ remains as low as possible and the mechanical power of the compressor and the temperature of the alcohol/water mixture G_(RD2) to be evaporated remains low.

When the rectification column RD₂ contains structured packings or unstructured packings these may be divided or in the form of an uninterrupted packing. However, typically at least two packings are provided, one packing above the feed point of the stream S_(RDS1)/the portion of S_(RDS1), in particular S_(RDS12), and one packing below the relevant feed point. If an unstructured packing is used, for example a random packing, the random packing elements are typically disposed on a suitable sieve tray or mesh tray.

S_(RDS1)/the portion of S_(RDS1) which is passed into RD₂ and which is preferably S_(RDS12) is at least partially liquid.

Passage thereof into RD₂ via a liquid compressor or a pump P is thus further preferred.

At the end of step (d) of the process according to the invention a mixture G_(RD2) comprising water and ROH is finally obtained in the rectification column RD₂. The composition of the mixture G_(RD2) results especially from the composition of the stream S_(RDS1)/the portion of the stream S_(RDS1), preferably S_(RDS12), which is passed into RD₂.

4.6 Step (e) of the Process According to the Invention

In step (e) of the process according to the invention the mixture G_(RD2) comprising water and ROH is at a pressure p₂ and a temperature T₂ separated into an ROH-comprising vapour stream S_(RDB2) at the top of RD₂ and a bottoms stream S_(RDS2) comprising water and optionally ROH at the bottom of RD₂.

With the exception of the proviso that p₁>p₂ the pressure p₂ in RD₂ may be chosen by those skilled in the art according to their knowledge of the art. It is preferably in the range between 1 bar and 20 bar, preferably 1 bar and 15 bar, more preferably 1 to 10 bar, yet more preferably 1.00 to 2.00 bar, yet more preferably 1.10 to 1.80 bar, yet more preferably 1.10 to 1.50 bar, wherein, simultaneously, p₁>p₂.

The temperature T₂ in RD₂ may be chosen by those skilled in the art according to their knowledge of the art. It is preferably in the range from 40° C. to 220° C., preferably from 60° C. to 190° C.

The separation according to step (e) of the process according to the Invention is a distillative separation of the alcohol/water mixture G_(RD2) as is known to a person skilled in the art.

Obtained at the bottom of the rectification column RD₂ is a stream S_(RDS2) which may comprise <1% by weight of alcohol based on the total mass of S_(RDS2).

Also obtained at the top of the rectification column RD₂ is the vapour stream S_(RDB2) comprising ROH. The preferred mass fraction of ROH in this vapour stream S_(RDB2) is ≥99% by weight, preferably ≥99.6% by weight, more preferably ≥99.9% by weight, in each case based on the total mass of S_(RDB2), wherein the remainder is especially water.

In a preferred embodiment of the present invention S_(RDB2) is at least partially employed as reactant stream S_(AE1) in the reactive rectification column RR_(A) and if step (a2) is performed alternatively or in addition employed as reactant stream S_(BE1) in the reactive rectification column RR_(B).

In step (e) the stream S_(RDS2), preferably the portion S_(RDS12), completely or partially passed into the second rectification column RD₂ in step (d) is subjected to distillative separation.

4.7 Pressure Management as a Characterizing Feature

The process according to the invention is characterized in that during operation of the rectification columns RD₁ (step (c)) and RD₂ (step (e)) a certain pressure ratio is established.

Accordingly, p₁>p₂, p₁>p_(3A) and in the cases where step (a2) is performed p₁>p_(3B).

It has surprisingly been found that maintaining these pressures allows the demand for energy to be supplied in the form of heating steam to be minimized and the majority of the energy required for the process to be covered through electricity.

It is yet more advantageous when in addition the pressures are established such that p_(3A)>p₂ and in cases where step (a2) is performed in addition p_(3B)>p₂. Establishing the pressures p_(3A) and p_(3B) in such a way reduces the altogether required energy demand compared to the case where p_(3A)<p₂/p_(3B)<p₂.

4.8 Characterizing Step (f): Enemy Transfer from S_(RDB2) to G_(RD1)

The step (f) of the process according to the Invention which is characterizing in addition to the pressure regime is that energy is transferred from S_(RDB1) to the mixture G_(RD2) in the second rectification column RD₂. According to the Invention “energy transfer” is in particular to be understood as meaning “heat transfer”.

This step (f) and the pressure regime according to the invention allow a particularly advantageous integration of the energy which would otherwise dissipate which makes it possible to cover a particularly large portion of the energy demand of the process through electricity instead of heating steam. This makes the process according to the invention particularly energy-efficient.

According to the invention the transfer of energy from S_(RDB1) to G_(RD2) in RD₂ may be effected in various ways familiar to those skilled in the art and preferably comprises heating G_(RD2) in RD₂ with S_(RDB1), for example via a heat transfer medium WT.

According to the invention in step (f) the energy is especially transferred from S_(RDB2) to G_(RD2) in RD₂ directly or indirectly, preferably directly.

4.8.1 Direct Energy Transfer from S_(RDB1) to G_(RD2) in RD₂

According to the invention “direct energy transfer from S_(RDB1) to G_(RD2) in RD₂” is to be understood as meaning that an energy transfer, preferably heating, of G_(RD2) in RD₂ with S_(RDB1) is effected such that G_(RD2) is contacted with S_(RDB1) without G_(RD2) undergoing mixing with S_(RDB1), thus transferring energy from S_(RDB1) to G_(RD2). However, direct energy transfer according to the invention is to be understood as also including cases where an energy transfer, preferably heating, of a stream S_(X) discharged from RD₂ with S_(RDB1) is effected without S_(RDB1) undergoing mixing with S_(X), thus transferring energy from S_(RDB1) to S_(X), and S_(X) is then passed back into RD₂ where it undergoes mixing with G_(RD2) in RD₂ and thus transfers the energy absorbed from S_(RDB1) to G_(RD2) in RD₂.

In a particular embodiment of the present invention S_(X) is selected from the group consisting of S_(RDS22), S_(RDX2).

Contacting without mixing is achieved by processes known to those skilled in the art, for example by contacting via a dividing wall made of metal, plastic etc., in particular in a heat exchanger WT, preferably a condenser K or evaporator V which is in particular selected from bottoms evaporators VS and intermediate evaporators VZ.

According to the invention it is preferable when direct energy transfer from S_(RDB1) to the mixture G_(RD2) in the second rectification column RD₂ is performed according to at least one of the steps (α-i), (α-ii), (α-iii), more preferably according to at least one of the steps (α-i), (α-ii).

(α-i) energy from S_(RDB1) is transferred to a portion S_(RDS22) of the bottoms stream S_(RDS2) discharged from RD₂ and S_(RDS22) is then recycled into RD₂. This step (α-i) also comprises embodiments in which energy is initially transferred from S_(RDB1), preferably via a heat exchanger WT, to the overall bottoms stream S_(RDS2) and the portion S_(RDS22) is only then separated from the bottoms stream S_(RDS2) and subsequently S_(RDS22) is recycled into RD₂.

(α-ii) at least one stream S_(RDX2) distinct from S_(RDB2) and S_(RDS2) comprising ROH and water is discharged from RD₂, energy is then transferred from S_(RDS1) to S_(RDX2), preferably via a heat exchanger WT, and S_(RDX2) is recycled into RD₂.

It is preferable when S_(RDX2) is withdrawn below the vapour stream S_(RDB2) on RD₂. S_(RDX2) is then especially selected from bottoms stream S_(RDX2S), intermediate stream S_(RDX2Z).

A bottoms stream S_(RDX2S) is a stream whose withdrawal point RD₂ is at the same height or below the withdrawal point of S_(RDS2). S_(RDX2S) may then be passed through a heat exchanger WT, in particular a bottoms evaporator VS, and energy transferred therein from S_(RDS1) to S_(RDX2S).

An intermediate stream S_(RDX2Z) is a stream whose withdrawal point on RD₂ is between the withdrawal points of S_(RDB2) and S_(RDS2). S_(RDX2Z) may then be discharged from RD₂ and passed through a heat exchanger WT, in particular an intermediate evaporator VZ, and energy transferred from S_(RDB1) to S_(RDX2Z) therein.

(α-iii) S_(RDB1) is passed through RD₂, thus transferring energy from S_(RDB1) to G_(RD2), preferably via a heat exchanger WT. Such an embodiment may be realized for example when S_(RDB1) is passed through the rectification column RD₂ through a conduit whose surface S_(RDB1) transfers energy to G_(RD2) in RD₂.

4.8.2 Indirect Energy Transfer from S_(RDB1) to G_(RD2) in RD₂

According to the invention “indirect energy transfer from S_(RDB1) to G_(RD2) in RD₂” is to be understood as meaning that an energy transfer, preferably heating, of G_(RD2) with S_(RDB1) is effected in RD₂ such that G_(RD2) is not directly contacted with S_(RDB1) but rather at least one additional, preferably precisely one additional, heat transfer medium W₁ distinct from G_(RD2) and S_(RDB1) is employed which during energy transfer from S_(RDB1) to G_(RD2) in RD₂ undergoes mixing neither with S_(RDB1) nor with G_(RD2) in RD₂. Energy is transferred from S_(RDB1) to the at least one heat exchanger W₁ without S_(RDB1) and the at least one heat exchanger W₁ undergoing mixing and then transferred from the at least one heat transfer medium W₁ to G_(RD2) in RD₂ without the at least one heat exchanger W₁ and G_(RD2) undergoing mixing.

Indirect energy transfer according to the invention is to be understood as also including cases where energy is transferred from S_(RDB1) to the at least one, preferably precisely one, heat transfer medium W₁ without S_(RDB1) and the at least one heat exchanger W₁ undergoing mixing and subsequently an energy transfer, preferably heating, of a stream S_(X) discharged from RD₂ with the at least one heat transfer medium W₁ is effected without the at least one heat transfer medium W₁ undergoing mixing with S_(X), thus transferring energy from the at least one heat transfer medium W₁ to S_(X), and S_(X) is then recycled into RD₂ where it undergoes mixing with G_(RD2) in RD₂ and thus transfers the energy absorbed by S_(RDB1) via the at least one heat transfer medium W₁ to G_(RD2) in RD₂.

In a particular embodiment of the present invention S_(X) is selected from the group consisting of S_(RDS22), S_(RDX2).

“At least one heat transfer medium W₁” comprises the cases where the energy of W₁ is first transferred to one or more further heat transfer media W₂, W₃, W₄, W₅ etc. distinct from G_(RD2) and S_(RDB1) and the last of these heat transfer media, referred to as “W_(Y)” is contacted with G_(RD2) in RD₁, thus transferring energy, preferably heat, from W_(Y) to G_(RD2) but without W_(Y) and G_(RD2) undergoing mixing. Energy, preferably heat, may likewise be transferred from W_(Y) to a stream S_(X) discharged from RD₂ without W_(Y) and S_(X) undergoing mixing and S_(X) subsequently recycled into RD₂ where it undergoes mixing with G_(RD2) in RD₂, thus transferring the energy absorbed by W_(Y) to G_(RD2) in RD₂.

The described contacting is in each case preferably performed in a heat exchanger WT, preferably a condenser K or evaporator V, which is especially selected from bottoms evaporators VS and intermediate evaporators VZ.

According to the invention it is preferable when indirect energy transfer from S_(RDB1) to the mixture G_(RD2) in the second rectification column RD₂ is performed according to at least one of the steps (β-i), (β-ii), (β-iii), more preferably according to at least one of the steps (β-i), (β-ii).

(β-i) a portion S_(RDS22) of the bottoms stream S_(RDS2) discharged from RD₂ is recycled into the second rectification column RD₂. Energy is transferred from S_(RDB1) to at least one heat transfer medium W_(i1) distinct from S_(RDS22) and then transferred from the at least one heat transfer medium W_(i1) to S_(RDS22) and S_(RDS22) is then recycled into RD₂:

This step (β-i) also comprises embodiments in which energy is initially transferred from the at least one, preferably precisely one, heat transfer medium W_(i1) distinct from S_(RDS22), preferably via a heat exchanger WT, to the overall bottoms stream S_(RDS2) and the portion S_(RDS22) is only then separated from the bottoms stream S_(RDS2) and subsequently S_(RDS22) is recycled into RD₂.

(β-ii) at least one stream S_(RDX2) distinct from S_(RDB2) and S_(RDS2) comprising ROH and water is discharged from RD₁. Energy is transferred from S_(RDB1) to at least one, preferably precisely one, heat transfer medium W_(ii1) distinct from S_(RDX2), preferably via a heat exchanger WT, and then transferred from the at least one heat transfer medium W_(ii1) to S_(RDX2) and S_(RDX2) is then recycled into RD₂.

It is preferable when S_(RDX2) is withdrawn below the vapour stream S_(RDB2) on RD₂. S_(RDX2) is then especially selected from bottoms stream S_(RDX2S), intermediate stream S_(RDX2Z).

A bottoms stream S_(RDX2S) is a stream whose withdrawal point on RD₂ is at the same height or below the withdrawal point of S_(RDS2). S_(RDX2S) may then be passed through a heat exchanger WT, in particular a bottoms evaporator VS, and energy transferred therein from S_(RDB1) to S_(RDX2S).

An intermediate stream S_(RDX2Z) is a stream whose withdrawal point on RD₂ is between the withdrawal points of S_(RDB2) and S_(RDS2). S_(RDX2Z) may then be discharged from RD₂ and passed through a heat exchanger WT, in particular an intermediate evaporator VZ, and energy transferred from S_(RDB1) to S_(RDX2Z) therein.

(β-iii) energy is transferred from S_(RDB1) to at least one heat transfer medium W_(iii1) distinct from G_(RD2) and the at least one heat transfer medium W_(iii1) is then passed through RD₂, thus transferring energy from the at least one heat transfer medium W_(iii1) to G_(RD2).

Such an embodiment may be realized for example when the at least one heat transfer medium W_(iii1) is passed through the rectification column RD₂ through a conduit whose surface transfers energy from the at least one heat transfer medium W_(iii1) to G_(RD2) in RD₂.

Employable heat transfer media W₁ W₂, W₃, W₄, W₅/at least one heat exchanger W_(i1)/at least one heat exchanger W_(ii1)/at least one heat exchanger W_(iii1) include any heat transfer media known to those skilled in the art. Such heat transfer media are preferably selected from the group consisting of water; alcohol-water solutions; salt-water solutions, also including ionic liquids such as for example LiBr solutions, dialkylimidazolium salts such as especially dialkylimidazolium dialkylphosphates; mineral oils, for example diesel oils, thermal oils such as for example silicone oils; biological oils such as for example limonene; aromatic hydrocarbons such as for example dibenzyltoluene. The most preferred heat transfer medium is water.

Salt-water solutions that may be used are also described for example in DE 10 2005 028 451 A1 and WO 2006/134015 A1.

4.9 Addition or Fresh Alcohol

The alcohol ROH is consumed in the process according to the invention and especially in a continuous process mode therefore requires replacement with fresh alcohol ROH.

Fresh alcohol is in particular added to at least one of the columns selected from rectification column RD₁, rectification column RD₂, reactive rectification column RR_(A) and if step (a2) is performed alternatively or in addition added to the reactive rectification column RR_(B).

In a preferred embodiment of the present invention a stream S_(XE1) distinct from S_(AE1) and S_(BE1) comprising ROH is accordingly added to at least one of the columns selected from rectification column RD₁, rectification column RD₂, reactive rectification column RR_(A) and if step (a2) is performed alternatively or in addition added to reactive rectification column RR_(B).

The introduction of the fresh alcohol ROH is effected, in particular, directly as reactant stream S_(AE1) comprising ROH into the reaction column RR_(A) or, in the embodiments in which step (a2) is carried out, into the reaction columns RR_(A) and RR_(B).

In the process according to the invention it is further preferable to employ the ROH-comprising vapour stream S_(RDB1) at least partially as reactant stream S_(AE1) in step (a1) and optionally as reactant stream S_(BE1) in step (a2). The vapour stream S_(RDB2) may alternatively or in addition be employed at least partially as reactant stream S_(AE1) in step (a1) and optionally as reactant stream S_(BE1) in step (a2).

In the particularly preferred embodiment in which S_(RDB1) and S_(RDB2) are employed at least partially as reactant stream S_(AE1), in step (a1) and optionally as reactant stream S_(BE1) in step (a2) S_(RDB1) and S_(RDB2) may be supplied to the respective reactive rectification column RR_(A)/RR_(B) separately from one another or first mixed with one another and then supplied to the respective reactive rectification column RR_(A)/RR_(B). S_(RDB1) and S_(RDB2) are preferably firstly mixed with one another and then supplied to the respective reactive rectification column RR_(A)/RR_(B).

In this preferred embodiment it is yet more preferable when the fresh alcohol ROH is added to one of the rectification columns RD₁ and RD₂, preferably RD₁.

When the fresh alcohol ROH is added to the rectification column RD₁ or RD₂ it is preferably supplied either in the reinforcing section of the respective rectification column or directly at the top of the respective rectification column. The optimal feed point depends on the water content of the employed fresh alcohol and also on the desired residual water content in the vapour stream S_(RDB1)/S_(RDB2). The higher the proportion of water in the employed alcohol and the higher the purity requirements of the vapour stream S_(RDB1)/S_(RDB2) the more advantageous is a feed of a number of theoretical trays below the top of the rectification column RD₁/RD₂. Up to 20 theoretical trays below the top of the rectification column RD₁/RD₂ and in particular 1 to 5 theoretical trays are preferred.

When the fresh alcohol ROH is added to the rectification column RD₁/RD₂ it is added at the top of the rectification column RD₁/RD₂ at temperatures up to boiling point, preferably at room temperature. The fresh alcohol may have a dedicated feed provided for it or else when a portion of the alcohol withdrawn at the top of the rectification column RD₁/RD₂ is recycled may be mixed therewith after condensation and supplied to the rectification column RD₁/RD₂ together. In this case it is particularly preferable when the fresh alcohol is added to a condensate container in which the alcohol condensed from the vapour stream S_(RDB1)/S_(RDB2) is collected.

FIGURES

FIG. 1

FIG. 1 shows a process according to the invention for producing alkali metal alkoxides with a corresponding interconnection of the rectification columns. Employed are a reactive rectification column (“reactive rectification column” is hereinbelow abbreviated to “reaction column”) RR_(A) <3A> at a pressure of p_(3A) and two rectification columns RD₁ <1> and RD₂ <2> at pressures of p₁ and p₂ respectively. Here, p₁>p_(3A)>p₂.

In RR_(A)<3A> NaOH (stream S_(AE2) <3A02>) is reacted with methanol (stream S_(AE1) <3A01>) to afford a crude product RP_(A) <3A07> comprising water, methanol, NaOH and sodium methoxide. At the lower end of RR_(A)<3A> a methanol-sodium methoxide mixture S_(AP) <3A04> is withdrawn. The bottoms evaporator VS_(3A) <3A06> at the lower end of the reaction column RR_(A)<3A> is used to adjust the concentration of the methoxide solution to the desired value in the resulting mixture S_(AP*) <3A08>. There may additionally be attached at the bottom of the reaction column RR_(A) <3A> a further evaporator, especially for startup of the reaction column RR_(A) <3A> (not shown).

At the top of RR_(A) <3A> a methanol-water mixture is withdrawn as vapour stream S_(AB) <3A03>. S_(AB) <3A03> is supplied to the first water/methanol column RD₁ <1>, wherein optionally S_(AB) <3A03> is at the top of the reaction column RR_(A) <3A> partially condensed in the condenser K_(RRA) <3A05> and recycled in liquid form as reflux to the top of RR_(A) <3A>. At least a portion of the vapour S_(AB) <3A03> is then passed through a compressor VD₃₁ <10>, thus increasing the pressure of the vapour S_(AB) <3A03> from p_(3A) to the pressure p₁.

A methanol/water mixture G_(RD1) <108> is thus obtained in the first rectification column RD₁ <1>. Methanol is distillative recovered as vapour S_(RDB1) <101> in this first water/methanol column RD₁ <1>. The methanol recovered as vapour stream S_(RDB1) <101> is at the withdrawal point <109> at the top of RD₁ <1> discharged therefrom and partially at the top of the rectification column RD₁ <1> condensed in the condenser K_(RD1) <102> and recycled in liquid form as reflux to the top of RD₁ <1>. The remaining portion of the methanol recovered as vapour S_(RDB1) <101> is for example via a throttle D₁₃ <11> decompressed to the pressure p₃ and introduced Into RR_(A) <3A> as methanol stream S_(AE1) <3A01>.

At the lower end (another term for “lower end of a rectification column” is “bottom of a rectification column”) of RD₁ <1> a bottoms stream S_(RDS1) <103> comprising water and methanol is discharged at the withdrawal point <110>. A first portion S_(RDS11) <104> of the stream S_(RDS1) <103> is supplied to a second water/methanol column RD₂ <2>, a second portion S_(RDS12) <105> of the stream S_(RDS1) <103> is via a bottoms evaporator recycled to VS_(RD1) <106> in RD₁ <1>. S_(RDS11) <104> is for example via a throttle D₁₂ <12> decompressed to the pressure p₂ before it is introduced Into RD₂ <2>.

A methanol/water mixture G_(RD2) <206> is thus obtained in the second rectification column RD₂ <2>. In the rectification column RD₂ <2> residues of methanol from S_(RDS11) <104> are separated from the water and distillatively recovered as vapour stream S_(RDB2) <201> at the top of RD₂ <2>. The methanol recovered as vapour stream S_(RDB2) <201> is at the withdrawal point <208> at the top of RD₂ <2> discharged therefrom and partially at the top of the rectification column RD₂ <2> condensed in the condenser K_(RD2) <203> and recycled in liquid form as reflux to the top of RD₂ <2>. The remaining portion of the methanol recovered as vapour S_(RDB2) <201> is passed through a compressor VD₂₃ <13>, thus compressed to the pressure p₃ and, together with the vapours S_(RDB1) <101> from RD₁ <1> decompressed to the pressure p₃, introduced as methanol stream S_(AE1) <3A01> into RR_(A) <3A>.

At the lower end of RD₂ <2> a bottoms stream S_(RDS2) <202> comprising water and optionally methanol is discharged at the withdrawal point <207>. A portion S_(RDS22) <222> of S_(RDS2) <202> is heated via a bottoms evaporator VS_(RD2) <204> and recycled into RD₂ <2>.

For the heating of the portion of the bottoms stream S_(RDS2) <202> which is recycled via VS_(RD2) <204> into RD₂ <2> the energy liberated upon condensation of S_(RDB1) <101> in the condenser K_(RD1) <102> at the top of the rectification column RD₁ <1> is utilized. Said energy is recycled to VS_(RD2) <204> as indicated by the dashed arrow <4>. The supply may be effected indirectly, i.e. using a heat transfer medium distinct from S_(RDB1) <101> and S_(RDS2) <202>, or else directly, i.e. through contacting of S_(RDB1) <101> with S_(RDS2) <202> in the condenser K_(RD1) <102> or bottoms evaporator VS_(RD2) <204>. In the case of direct contacting it is sufficient to employ only the condenser K_(RD1) <102> and omit the bottoms evaporator VS_(RD2) or to employ only the bottoms evaporator VS_(RD2) <204> and omit the condenser K_(RD1) <102> and then in each case pass both streams S_(RDB1) <101> with S_(RDS2) <202> through the condenser K_(R1) <102> or the bottoms evaporator VS_(RD2) <204> such that energy, preferably heat, is transferred from S_(RDB1) <101> to S_(RDS2) <202>.

FIG. 2

FIG. 2 shows a further process according to the invention for producing alkali metal alkoxides. This differs from the process shown in FIG. 1 in terms or the pressures in the respective columns. In the embodiment shown in FIG. 1 p₁>p_(3A)>p₂ while in the embodiment shown in FIG. 2 p₁>p₂>p_(3A). This different pressure regime makes the compressor VD₂₃ <13> unnecessary and a throttle D₂₃ <14> for example is attached. The throttle D₂₃ <14> decompresses the vapour stream S_(RDB2) <201> from p₂ to the pressure p_(3A) while in the embodiment according to FIG. 1 the compressor VD₂₃ <13> increases it from p₂ to the pressure p_(3A).

FIG. 3

FIG. 3 shows one embodiment of a process not according to the invention for producing alkali metal alkoxides with a corresponding interconnection of the reactive rectification and rectification columns. Employed here, similarly to the embodiments described in FIGS. 1 and 2 , are a reactive rectification column RR_(A) <3A> at a pressure of p_(A) and two rectification columns RD₁ <1> and RD₂ <2> having pressures of p₁ and p₂ respectively. Here, p₂>p₁>p_(3A). The setup shown in FIG. 3 corresponds to the setup shown in FIG. 2 with the following exceptions:

1. Arranged at the rectification column RD₁ <1> next to the bottoms evaporator VS_(RD1) <106> is an intermediate evaporator VZ_(RD1) <107> which may be used to supply energy to the mixture G_(RD1) <108> in RD₁ <1>. To this end the mixture G_(RD1) <108> is at a withdrawal point <111> discharged from the rectification column RD₁ <1> as stream S_(RDX1) <112>. S_(RDX1) <112> is heated in VZ_(RD1) <107> and recycled into the rectification column RD₁ <1>. A corresponding intermediate evaporator may also be attached to RD₂ <2> in the embodiments according to examples 1 and 2. 2. The throttle D₁₂ <12> is on account of the different pressures in the rectification columns RD₁ <1> and RD₂ <2> (p₂>p₁) replaced by a pump P <15>. The reason for this difference is that the pressure of S_(RDS11) <104>, when this stream is passed into RD₂ <2>, is according to the invention increased to p₂. 3. In an optional embodiment additional methanol as stream S_(XE1) <205> is via the reflux at the rectification column RD₂ <2> added thereto. 4. The energy liberated upon condensation of the vapour S_(RDB2) <201> at the top of RD₂ <2> is via the intermediate evaporator VZ_(RD1) <107> transferred to S_(RDX1) <112> and after reintroduction of S_(RDX1) <112> into RD₁ <1> transferred from S_(RDX1) <112> to the mixture G_(RD1) <108> present in RD₁ <1>. Alternatively or in addition, energy liberated upon condensation of the vapour S_(RDB2) <201> at the top of RD₂ <2> is via the bottoms evaporator VS_(RD1) <106> transferred to the portion S_(RDS12) <105> of the stream S_(RDS1) <103>. Once S_(RDS12) <105> is recycled into RD₁ <1> it transfers the energy to the mixture G_(RD1) <108> present in RD₁ <1>. The energy flow is shown by the dashed arrow <4>.

In the case of direct contacting it is sufficient to employ only the condenser K_(RD2) <203> and omit the bottoms evaporator VS_(RD1) or to employ only the bottoms evaporator VS_(RD1) <106> and omit the condenser K_(RD2) <203> and then in each case pass both streams S_(RDB2) <201> with S_(RDS12) <105> through the condenser K_(RD2) <203> or the bottoms evaporator VS_(RD1) <106> such that energy, preferably heat, is transferred from S_(RDS2) <201> to S_(RDS12) <105>.

FIG. 4

FIG. 4 shows a further embodiment of a process not according to the invention for producing alkali metal alkoxides with a corresponding interconnection of the rectification columns. Employed here, similarly to the embodiments described in FIGS. 1 and 2 , are a reactive rectification column RR_(A) <3A> at a pressure of p_(3A) and two rectification columns RD₁ <1> and RD₂ <2> having pressures of p₁ and p₂ respectively. Here, p₂>p_(3A)>p₁. The setup shown in FIG. 4 corresponds to the setup shown in FIG. 3 with the exception that the pressure p_(3A)>p₁. This allows the compressor VD₃₁ <10> to be omitted while the throttle D₁₃ <11> is replaced by the compressor VD₁₃ <16>.

5. EXAMPLES

5.1 Example 1 (Inventive)

The setup according to example 1 corresponds to the two-column interconnection according to FIG. 1 , wherein p₁>p_(3A)>p₂.

A stream S_(AE2) <3A02> of aqueous NaOH (50% by weight) of 5 t/h is supplied to the top of a reaction column RR_(A) <3A> at 25° C. A vaporous methanol stream S_(AE1) <3A01> of 70.2 t/h is supplied in countercurrent above the bottom of the reaction column RR_(A) <3A>. The reaction column RR_(A) <3A> is operated at a pressure p_(3A) of 1.6 bar. At the bottom of the column RR_(A) <3A> a virtually water-free product stream S_(AP*) <3A08> of 10.8 t/h is withdrawn (30% by weight sodium methoxide in methanol). At the evaporator VS_(3A) <3A06> of the reaction column RR_(A) <3A> about 0.7 MW of heating power are introduced using heating steam. A vaporous methanol-water stream S_(AB) <3A03> is withdrawn at the top of the reaction column RR_(A) <3A>. A portion of this stream is via a condenser K_(RRA) <3A05> recycled to the reaction column RR_(A)<3A> and the remaining portion (64.4 t/h) compressed in a compressor VD₃₁ <10> to 7.1 bar, wherein about 4 MW of compressor power are necessary, and supplied to a first rectification column RD₁ <1>. The rectification column RD₁ <1> is operated at p₁=˜7 bar. At the top of the rectification column RD₁ <1> a liquid fresh methanol stream of 9.5 t/h is supplied (not shown in FIG. 1 ) and vaporous methanol stream S_(RDB1) <101> is withdrawn. A portion of S_(RDB1) <101> is via the condenser K_(RD1) <102> recycled into column RD₁ <1>. The remaining portion of SRDS <101> (42.9 t/h) is suppled to the reaction column RR_(A) <3A>. Condenser K_(RD1) <102> of column RD₁ <1> which is simultaneously the evaporator VS_(RD2) of the second rectification column RD₂ <2> provides the heating power for the column RD₂ <2>. The embodiment according to example 1 utilized direct contacting where the condenser K_(RD1) <102> is simultaneously employed as bottoms evaporator VS_(RD2) <204>.

Discharged at the bottom of the rectification column RD₁ <1> is a liquid stream of a water-methanol mixture S_(RDS1) <103> of which a portion S_(RDS12) <104> of 30.9 t/h is passed into the rectification column RD₂ <2> and the remaining portion of the stream SROS <103> is recycled as S_(RDS11) <105> into RD₁ <1>. At the evaporator VS_(RD1) <106> of the rectification column RD₁ <1> about 5.4 MW of heating power are introduced via heating steam.

The rectification column RD₂ <2> is operated at a pressure p₂ of 1.1 bar. Withdrawn at the top of the rectification column RD₂ <2> is a vaporous methanol stream S_(RDB2) <201>. A portion of S_(RDB2) <201> is via the condenser K_(RD2) <203> recycled into column RD₂ <2>. The remaining portion of S_(RDB2) <201> (27.3 t/h) is supplied to the reaction column RR_(A) <3A>. This portion of the vaporous stream S_(RDB2) <201> is compressed to 2 bar in a compressor VD₂₃ <13>, wherein about 0.6 MW of compressor power are necessary. Discharged at the bottom of the rectification column RD₂ <2> is a liquid stream of water S_(RDS2) <202> (contaminated with 500 ppmw of methanol) of 3.7 t/h. For evaporation at the rectification column RD₂ <2> (since direct heat integration is effected the function of the bottoms evaporator VS_(RD2) <204> shown in FIG. 1 is co-assumed by the condenser K_(RD1) <102>) about 14.8 MW of heating power are introduced to a portion S_(RDS22) <222> of S_(RDS2) <202> via heat integration with the column RD₁ <1>.

The respective, non-recycled portions of the vaporous methanol streams S_(RDB1) <101> and S_(RDB2) <201> withdrawn at the tops of RD₁ <1> and RD₂ <2> are mixed and recycled to the bottom of reaction column RR_(A) <3A>.

Altogether in this example about 6.1 MW of heating power via heating steam and about 4.6 MW of electrical power (compressor power) are required and must be externally provided.

5.2 Example 2 (inventive)

The setup according to example 2 corresponds to the two-column interconnection according to FIG. 2 , wherein p₁>p₂>p_(3A).

A stream S_(AE2) <3A02> of aqueous NaOH (50% by weight) of 5 t/h is supplied to the top of a reaction column RR_(A) <3A> at 25° C. A vaporous methanol stream S_(AE1) <3A01> of 70.2 t/h is supplied in countercurrent above the bottom of the reaction column RR_(A) <3A>.

The reaction column RR_(A) <3A> is operated at a pressure p_(3A) of 1.1 bar. At the bottom of the column RR_(A) <3A> a virtually water-free product stream S_(AP*) <3A08> of 10.8 t/h is withdrawn (30% by weight sodium methoxide in methanol). At the evaporator VS_(3A) <3A06> of the reaction column RR_(A)<3A> about 2.4 MW of heating power are introduced using heating steam. A vaporous methanol-water stream S_(AB) <3A03> is withdrawn at the top of the reaction column RR_(A) <3A>. A portion of this stream is via a condenser K_(RRA) <3A05> recycled to the reaction column RR_(A) <3A> and the remaining portion (64.4 t/h) compressed in a compressor VD₃₁ <10> to 9 bar, wherein about 5.8 MW of compressor power are necessary, and supplied to a first rectification column RD₁ <1>. The rectification column RD₁ <1> is operated at p₁=˜8.9 bar. At the top of the rectification column RD₁ <1> a liquid fresh methanol stream of 9.5 t/h is supplied (not shown in FIG. 2 ) and vaporous methanol stream S_(RDB1) <101> is withdrawn. A portion of S_(RDB1) <101> is via the condenser K_(RD1) <102> recycled into column RD₁ <1>. The remaining portion of S_(RDB1) <101> (42.9 t/h) is supplied to the reaction column RR_(A) <3A>. Condenser K_(RD1) <102> of column RD₁ <1> which is simultaneously the evaporator VS_(RD2) <204> of the second rectification column RD₂ <2> provides the heating power for the column RD_(3<2)>. The embodiment according to example 2 utilized direct contacting where the condenser K_(RD1) <102> is simultaneously employed as bottoms evaporator VS_(RD2) <204>.

Discharged at the bottom of the rectification column RD₁ <1> is a liquid stream of a water-methanol mixture S_(RDS1) <103> of which a portion S_(RDS12) <104> of 31.9 t/h is passed into the rectification column RD₂ <2> and the remaining portion of the stream S_(RDS1) <103> is recycled as S_(RDS11) <105> Into RD₁ <1>. At the evaporator VS_(RD1) <106> of the rectification column RD₁ <1> about 5.2 MW of heating power are introduced via heating steam.

The rectification column RD₂ <2> is operated at a pressure p₂ of 1.5 bar. Withdrawn at the top of the rectification column RD₂ <2> is a vaporous methanol stream S_(RDB2) <201>. A portion of S_(RDB2) <201> is via the condenser K_(RD2) <203> recycled into column RD₂ <2>. The remaining portion of S_(RDB2) <201> (28.2 t/h) is supplied to the reaction column RR_(A) <3A>. Discharged at the bottom of the rectification column RD₂ <2> is a liquid stream of water S_(RDS2) <202> (contaminated with 500 ppmw of methanol) of 3.7 t/h. For evaporation at the rectification column RD₂ <2> (since direct heat integration is effected the function of the bottoms evaporator VS_(RD2) <204> shown in FIG. 1 is co-assumed by the condenser K_(RD1) <102>) about 15.9 MW of heating power are introduced to a portion S_(RDS22) <222> of S_(RDS2) <202> via heat integration with the column RD₁ <1>. The respective, non-recycled portions of the vaporous methanol streams S_(RDB1) <101> and S_(RDB2) <201> withdrawn at the tops of RD₁ <1> and RD₁ <2> are mixed and recycled to the bottom of reaction column RR_(A) <3A>.

Altogether in this example about 7.6 MW of heating power via heating steam and about 5.8 MW of electrical power (compressor power) are required and must be externally provided.

5.3 Example 3 (Noninventive)

The setup according to example 3 corresponds to the two-column interconnection according to FIG. 3 , wherein p₂>p₁>p_(3A). The intermediate evaporator VZ_(RD1) <107> with the stream S_(RDX1) <112> withdrawn at the withdrawal point <111> shown in FIG. 3 is likewise omitted in the setup according to example 3. The condenser K_(RD2) <203> is also simultaneously the bottoms evaporator VS_(RD1) <106>. The fresh methanol stream S_(XE1) <205> is in FIG. 3 supplied to the rectification column RD₂ <2> but in the setup according to example 3 supplied to RD₁ <1>.

A stream S_(AE2) <3A02> of aqueous NaOH (50% by weight) of 5 t/h is supplied to the top of a reaction column RR_(A) <3A> at 25° C. A vaporous methanol stream S_(AE1) <3A01> of 70.2 t/h is supplied in countercurrent above the bottom of the reaction column RR_(A) <3A>. The reaction column RR_(A) <3A> is operated at a pressure p_(3A) of 1.1 bar. At the bottom of the column RR_(A) <3A> a virtually water-free product stream S_(AP*) <3A08> of 10.8 t/h is withdrawn (30% by weight sodium methoxide in methanol). At the evaporator VS_(3A) <3A06> of the reaction column RR_(A) <3A> about 1.4 MW of heating power are introduced using heating steam. A vaporous methanol-water stream S_(AB) <3A03> is withdrawn at the top of the reaction column RR_(A) <3A>. A portion of this stream is via a condenser K_(RRA) <3A05> recycled to the reaction column RR_(A) <3A> and the remaining portion (64.4 t/h) compressed in a compressor VD₃₁ <10> to 1.7 bar, wherein about 1.1 MW of compressor power are necessary, and supplied to a first rectification column RD₁ <1>. The rectification column RD₁ <1> is operated at p₁=˜1.5 bar. At the top of the rectification column RD₁ <1> a liquid fresh methanol stream S_(XE1) <205> of 9.5 t/h is supplied (shown at the top of the rectification column RD₂ <2> in FIG. 3 ) and vaporous methanol stream S_(RDB1) <101> is withdrawn. A portion of S_(RDB1) <101> is via the condenser K_(RD1) <102> recycled into column RD₁ <1>. The remaining portion of S_(RDB1) <101> (58.8 t/h) is supplied to the reaction column RR_(A) <3A>. The embodiment according to example 3 utilized direct contacting where the condenser K_(RD2) <203> simultaneously serves as bottoms evaporator VS_(RD1) <106>.

Discharged at the bottom of the rectification column RD₁ <1> is a liquid stream of a water-methanol mixture S_(RDS1) <103> of which a portion S_(RDS12) <104> or 17 t/h is passed into the rectification column RD₂ <2> and the remaining portion of the stream S_(RDS1) <103> is recycled as S_(RDS11) <105> into RD₁ <1>.

The pressure of the discharged stream S_(RDS12) <104> is in a pump P <15> increased to 9 bar and the stream S_(RDS12) <104> supplied to the second rectification column RD₂ <2>. The rectification column RD₂ <2> is operated at a pressure p₂ of 8.9 bar. In the condenser K_(RD2) <203> of column RD₂ <2> which is simultaneously the evaporator of the column RD₁ <1> about 8.2 MW of heating power are provided for the column RD₁ <1>. Withdrawn at the top of the rectification column RD₂ <2> is a vaporous methanol stream S_(RDB2) <201>. A portion of S_(RDB2) <201> is via the condenser K_(RD2) <203> recycled into column RD₂ <2>. The remaining portion of S_(RDB2) <201> (13.4 t/h) is supplied to the reaction column RR_(A) <3A>. Discharged at the bottom of the rectification column RD₂ <2> is a liquid stream of water (contaminated with 500 ppmw of methanol) of 3.7 t/h. At the evaporator VS_(RD2) <204> of the rectification column RD₂ <2> about 12.9 MW of heating power are introduced using heating steam.

The respective, non-recycled portions of the vaporous methanol streams S_(RDB1) <101> and S_(RDB2) <201> withdrawn at the tops of RD₁ <1> and RD₂ <2> are mixed, decompressed and recycled to the bottom of reaction column RR_(A) <3A>.

Altogether in this example about 14.3 MW of heating power via heating steam and about 1.1 MW of electrical power (compressor power) are required and must be externally provided.

5.4 Example 4 (Noninventive)

The setup according to example 4 corresponds to the two-column interconnection according to FIG. 4 , wherein p₂>p_(3A)>p₁. The intermediate evaporator VZ_(RD1) <107> with the stream S_(RDX1) <112> withdrawn at the withdrawal point <111> shown in FIG. 4 is likewise omitted in the setup according to example 4. The condenser K_(RD2) <203> is also simultaneously the bottoms evaporator VS_(RD1) <106>. The fresh methanol stream S_(XE1) <205> is in FIG. 4 supplied to the rectification column RD₂ <2> but in the setup according to example 4 supplied to RD₁ <1>.

A stream S_(AE2) <3A02> of aqueous NaOH (50% by weight) of 5 t/h is supplied to the top of a reaction column RR_(A) <3A> at 25° C. A vaporous methanol stream S_(AE1) <3A01> of 70.2 t/h is supplied in countercurrent above the bottom of the reaction column RR_(A) <3A>. The reaction column RR_(A) <3A> is operated at a pressure p_(3A) of 1.8 bar. At the bottom of the column RR_(A) <3A> a virtually water-free product stream S_(AP*) <3A08> of 10.8 t/h is withdrawn (30% by weight sodium methoxide in methanol). At the evaporator VS_(3A) <3A06> of the reaction column RR_(A) <3A> about 0.8 MW of heating power are introduced using heating steam. A vaporous methanol-water stream S_(AB) <3A03> is withdrawn at the top of the reaction column RR_(A) <3A>. A portion of this stream is via a condenser K_(RRA) <3A05> recycled to the reaction column RR_(A) <3A> and the remaining portion (64.4 t/h) is supplied to a first rectification column RD₁ <1>. The rectification column RD₁ <1> is operated at p₁=˜1.1 bar. At the top of the rectification column RD₁ <1> a liquid fresh methanol stream S_(XE1) <205> of 9.5 t/h is supplied (shown at the top of the rectification column RD₂ <2> in FIG. 4 ) and vaporous methanol stream S_(RDB1) <101> is withdrawn.

A portion of S_(RDB1) <101> is via the condenser K_(RD1) <102> recycled into column RD₁ <1>. The remaining portion of S_(RDB1) <101> (55 t/h) is compressed to 2 bar in a compressor VD₁₃ <16>, wherein about 1.2 MW of compressor power are required, and supplied to the reaction column RR_(A) <3A>. Discharged at the bottom of the rectification column RD₁ <1> is a liquid stream of a water-methanol mixture S_(RDS1) <103> of which a portion S_(RDS12) <104> of 18.9 t/h is passed into the rectification column RD₂ <2> and the remaining portion of the stream S_(RDS1) <103> is recycled as S_(RDS11) <105> into RD₁ <1>.

The pressure or the discharged stream S_(RDS12) <104> is in a pump P <15> increased to 3.4 bar and the stream supplied to the second rectification column RD₂ <2>. The rectification column RD₂ <2> is operated at a pressure p₂ of 3.2 bar. In the condenser K_(RD2) <203> of column RD₂ <2> which is simultaneously the evaporator of the column RD₁ <1>, about 6.3 MW of heating power are provided for the column RD₁ <1>. Withdrawn at the top of the rectification column RD₂ <2> is a vaporous methanol stream S_(RDB2) <201>. A portion of S_(RDB2) <201> is via the condenser K_(RD2) <203> recycled into column RD₂ <2>. The remaining portion of S_(RDB2) <201> (15.2 t/h) is supplied to the reaction column RR_(A) <3A>. Discharged at the bottom of the rectification column RD₂ <2> is a liquid stream of water (contaminated with 500 ppmw of methanol) of 3.7 t/h. At the evaporator VS_(RD2) <204> or the rectification column RD₂ <2> about 11.4 MW of heating power are introduced using heating steam.

The respective, non-recycled portions of the vaporous methanol streams S_(RDB1) <101> and S_(RDB2) <201> withdrawn at the tops of RD₁ <1> and RD₂ <2> are mixed and recycled to the bottom of reaction column RR_(A) <3A>.

Altogether in this example about 12.2 MW of heating power via heating steam and about 1.2 MW of electrical power (compressor power) are required and must be externally provided.

5.5 Result

Comparison of the proportion of heating steam and electrical current required to cover the energy demand in the inventive and noninventive examples reveals that the inventive process surprisingly makes it possible to cover a large proportion of the energy demand through electrical energy and to minimize the proportion of the power to be provided through heating steam. 

The invention claimed is:
 1. A process for producing at least one alkali metal alkoxide of formula M_(A)OR, wherein R is a C₁ to C₆ hydrocarbon radical and wherein M_(A) is sodium or potassium, the process comprising: (a1) reacting a reactant stream S_(AE1) comprising ROH with a reactant stream S_(AE2) comprising M_(A)OH, in countercurrent at a pressure p_(3A) and a temperature T_(3A) in a first reactive rectification column RR_(A), to afford a crude product RP_(A) comprising M_(A)OR, water, the ROH, and the M_(A)OH, wherein R is a C₁ to C₆ hydrocarbon radical and M_(A) is sodium or potassium, and withdrawing a bottoms product stream S_(AP) comprising the ROH and the M_(A)OR at a lower end of the first reactive rectification column RR_(A), and withdrawing a vapour stream S_(AB) comprising the water and the ROH at an upper end of the first reactive rectification column RR_(A), and (a2) optionally, simultaneously with and spatially separate from (a1), reacting a reactant stream S_(BE1) comprising ROH with a reactant stream S_(BE2) comprising M_(B)OH, in countercurrent at a pressure p_(3B) and a temperature T_(3B) in a second reactive rectification column RR_(B), to afford a crude product RP_(B) comprising M_(B) OR, water, the ROH, and the M_(B)OH, wherein M_(B) is sodium or potassium, and withdrawing a bottoms product stream S_(BP) comprising the ROH and the M_(B)OR at a lower end of the second reactive rectification column RR_(B), and withdrawing a vapour stream S_(BB) comprising the water and the ROH at an upper end of the second reactive rectification column RR_(B), (b) passing the vapour stream S_(AB) into a first rectification column RD₁, and if (a2) is performed, the vapour stream S_(BB) is also passed into the first rectification column RD₁ in admixture with the vapour stream S_(AB) or separately from the vapour stream S_(AB), to obtain a mixture G_(RD1) comprising water and ROH in the first rectification column RD₁, (c) separating the mixture G_(RD1) in the first rectification column RD₁ at a pressure p₁ and a temperature T₁, into an ROH-comprising vapour stream S_(RDB1) at an upper end of the first rectification column RD₁, and a bottoms stream S_(RDS1) comprising water and ROH at a lower end of the first rectification column RD₁, (d) passing the bottoms stream S_(RDS1) completely or partially into a second rectification column RD₂, to obtain a mixture G_(RD2) comprising water and ROH in the second rectification column RD₂, (e) separating the mixture G_(RD2) at a pressure p₂ and a temperature T₂ into an ROH-comprising vapour stream S_(RDB2) at a top of the second rectification column RD₂, and a bottoms stream S_(RDS2) comprising water at a lower end of the second rectification column RD₂, and (f) transferring energy from the vapour stream S_(RDB1) to the mixture G_(RD2) in the second rectification column RD₂, wherein p₁>p₂, p₁>p_(3A), and wherein if (a2) is performed, p₁>p_(3B).
 2. The process according to claim 1, wherein in (f), energy is directly transferred from the vapour stream S_(RDB1) to the mixture G_(RD2).
 3. The process according to claim 2, wherein at least one of (α-i), (α-ii), and/or (α-iii) is performed: (α-i) energy from the vapour stream S_(RDB1) is transferred to a portion S_(RDS22) of the bottoms stream S_(RDS2) discharged from the second rectification column RD₂, and the portion S_(RDS22) is then recycled into the second rectification column RD₂; (α-ii) at least one stream S_(RDX2), distinct from the vapour stream S_(RDB2) and the bottoms stream S_(RDS2), comprising ROH and water is discharged from the second rectification column RD₂, and energy is then transferred from the vapour stream S_(RDB1) to the at least one stream S_(RDX2), and the at least one stream S_(RDX2) is recycled into the second rectification column RD₂; and/or (α-iii) the vapour stream S_(RDB1) is passed through the second rectification column RD₂, thus transferring energy from the vapour stream S_(RDB1) to the mixture G_(RD2).
 4. The process according to claim 3, wherein the at least one stream S_(RDX2) is withdrawn below the vapour stream S_(RDB2) on the second rectification column RD₂.
 5. The process according to claim 1, wherein in (f), energy is indirectly transferred from the vapour stream S_(RDB1) to the mixture G_(RD2).
 6. The process according to claim 5, wherein at least one of (β-i), (β-ii), and/or (β-iii) is performed: (β-i) a portion S_(RDS22) of the bottoms stream S_(RDS2) discharged from the second rectification column RD₂ is recycled into the second rectification column RD₁, wherein energy is transferred from the vapour stream S_(RDB1) to at least one heat transfer medium W_(i1), distinct from the portion S_(RDS22), and then transferred from the at least one heat transfer medium W_(i1) to the portion S_(RDS22), and the portion S_(RDS22) is then recycled into the second rectification column RD₂; (β-ii) at least one stream S_(RDX2), distinct from the vapour stream S_(RDB2) and the bottoms stream S_(RDS2), comprising ROH and water is discharged from the second rectification column RD₂, and energy is transferred from the vapour stream S_(RDB1) to at least one heat transfer medium W_(ii1), distinct from the at least one stream S_(RDX2), and then transferred from the at least one heat transfer medium W_(ii1) to the at least one stream S_(RDX2), and the at least one stream S_(RDX2) is then recycled into the second rectification column RD₂; and/or (β-iii) energy is transferred from the vapour stream S_(RDB1) to at least one heat transfer medium W_(iii1), distinct from the mixture G_(RD2), and the at least one heat transfer medium W_(iii1) is then passed through the second rectification column RD₂, thus transferring energy from the at least one heat transfer medium W_(iii1) to the mixture G_(RD2).
 7. The process according to claim 6, wherein each of the at least one heat transfer medium W_(i1), the at least one heat transfer medium W_(ii1), and the at least one heat transfer medium W_(iii1) is water.
 8. The process according to claim 6, wherein the at least one stream S_(RDX2) is withdrawn below the vapour stream S_(RDB2) on the second rectification column RD₂.
 9. The process according to claim 1, wherein the vapour stream S_(RDB2) is at least partially employed as the reactant stream S_(AE1) in the first reactive rectification column RR_(A), and if (a2) is performed, the vapour stream S_(RDB2) is alternatively or in addition employed as the reactant stream S_(BE1) in the second reactive rectification column RR_(B).
 10. The process according to claim 1, wherein the vapour stream S_(RDB1) is at least partially employed as the reactant stream S_(AE1) in the first reactive rectification column RR_(A), and if (a2) is performed, the vapour stream S_(RDB1) is alternatively or in addition employed as the reactant stream S_(BE1) in the second reactive rectification column RR_(B).
 11. The process according to claim 1, wherein a stream S_(XE1), distinct from the reactant stream S_(AE1) and the reactant stream S_(BE1), comprising ROH is added to at least one of the columns selected from the group consisting of the first rectification column RD₁, the second rectification column RD₂, and the first reactive rectification column RR_(A), and if (a2) is performed, the stream S_(XE1) is alternatively or in addition added to the second reactive rectification column RR_(a).
 12. The process according to claim 1, wherein R is methyl or ethyl.
 13. The process according to claim 1, wherein (a2) is performed.
 14. The process according to claim 1, wherein p_(3A)>p₂, and wherein if (a2) is performed, p_(3B)>p₂.
 15. The process according to claim 1, wherein the bottoms stream S_(RDS2) comprises water and ROH.
 16. The process according to claim 1, wherein the process is carried out continuously. 